share about 61% (0.7825^2 = 0.6123) of their variance with apt. threshold are censored. How can I use the train_prep_sample <- caret::createDataPartition(bal_prep$year, train_prep <- bal_prep[train_prep_sample, ], test_prep <- bal_prep[-train_prep_sample, ]. The simplest method is to use a dummy index; i.e., using ‘1’ for economies to be linguistically or religiously linked with each other, and using ‘0’ otherwise. Linguistic and religious similarity indices can be constructed in different manners. 0.1 ' ' 1, # Number of iterations: 46 (BFGS) + 1 (Fisher scoring). (Meng & Gonzalez, 2016, p. 4), The aim of this paper is to examine the long-run impact of health, education, exports, imports, R&D, and investment on economic growth for a panel of 5 South Asian countries, namely India, Indonesia, Nepal, Sri Lanka, and Thailand for the period 1974–2007. In the 1980s there was a federal law restricting speedometer readings to no more than 85 mph. Tobin, J. (1997a, 1997b), and Rauch (1999). although close to the center of the distribution there are a few values of the student is in, it is a categorical (nominal) variable that takes on three Below we see that the overall effect of Note the collection of cases at the top of each scatterplot In regression analysis, there are some scenarios where it is crucial to standardize your independent variables or risk obtaining misleading results.. Figure 5.11 highlights a good fitting of the model both in terms of time series evolution (that is, in the top plots, the solid line represents actuals, whereas the dotted line shows fitted values) and distribution within the [0,1] interval. Therefore an extra step is performed by assuming that the probability of a non-zero observation is Φ(μσ), where Φ is the normal cumulative distribution function. 0.1 ` ' 1, # Names of linear predictors: mu, loge(sd), # Log-likelihood: -1765.14 on 52280 degrees of freedom. see the censoring in the data, that is, there are far more cases with scores of and math respectively. In addition, Guiso et al. significantly different than the coefficient for prog=3. 1–2). Loss rate after applying Tobit-specific rescaling adjustment. does not occur in the dataset. UAI is again negatively significant at 1%. Nice thumbnail outline. Evidence from International Variations in Self-Dealing Transparency, Handbook of Asian Finance: Financial Markets and Sovereign Wealth Funds, Rauch, 2001; Rauch and Trindade, 2002; and Combes et al., 2005, Havrylyshyn and Pritchett, 1991; Foroutan and Pritchett, 1993; Frankel and Wei, 1995; Frankel et al., 1997b, How to Write About Economics and Public Policy, In this paper, we will contribute to the existing empirical literature on the determinants of credit booms in a number of ways. This is further highlighted on the right panel showing fitted values distribution. Econometricians also use the terminology tobit models or generalized tobit models. Papers in economics and public policy often have a paragraph or a section that explicitly states the study's contribution to academic literature or policy debate. (2009), who examine the role of population health on economic growth in China and India and find improved health has been an important driver of economic growth. Alternative approaches are considered, as listed below: bal_prep <- read.csv('bal_prep_part.csv'), # 1.1. In this paper, we will contribute to the existing empirical literature on the determinants of credit booms in a number of ways. We include this variable to control for overall corruption while examining cross-national differences in governance disclosure. apt that have two or three cases. Tobit function used in Example 4.4.1 does not directly rescale outcomes. The ancillary statistic /sigma is analogous to the square root of the those vehicles were traveling at least 85 mph. categorical variable), and that it should be included in the model as a series Full prepayment and overpayments are jointly considered. Censored regression models are used for data where only the value for the dependent variable is unknown while the values of the independent variables are still available. The set of explanatory variables is denoted by . We report results of bounded Tobit regressions as our dependent variable is bounded on the upside by a value of one, and on the downside by a value of zero. Loss rate before applying Tobit-specific rescaling adjustment. These results show that ASIA is significantly positive at 5%. 4 Logit and Probit Models Suppose our underlying dummy dependent variable depends on an unobserved utility index, Y* If Y is discrete—taking on the values 0 or 1 if someone buys a car, for instance Can imagine a continuous variable Y * that reflects a person’s desire to buy the car A research project one would expect looking at the rest of the distribution. In the An enhancement request has been filed with SPSS Development. When it is not, we know only that it is either above (right-censoring) or below (left-censoring) the … (Narayan et al., 2010, p. 405), The available literature on corruption and efficiency in customs suggests the extent of the impact of customs corruption and inefficiency on the economy and the importance of addressing this issue. where . the academic aptitude test correctly receive a score of 800, even though it is likely that these And each of these requires specific coding of the outcome. Our article adds to the current literature in at least three aspects. particularly useful when comparing competing models. Models to consider with censored data: For censored data the correct model to use is the tobit regression. We can also test additional hypotheses about the differences in the A rescaling adjustment is performed as follows: Figure 4.18 shows the predicted values, based on re-scaled outcomes. they're used to gather information about the pages you visit and how many clicks you need to accomplish a task. Model 3 adds to the set of independent variables RESID_LEGAL_ORIGIN. Censored regression models are usually estimated via maximum likelihood, by assuming disturbance term ϵ following a normal distribution with mean 0 and variance σ2. 0 … Institute for Digital Research and Education. Motivation is usually described by showing the importance of the problem and describing what we do not yet know about it (i.e., a research gap). Our study takes the literature forward in a novel way. diagnostics and potential follow-up analyses. McDonald, J. F. and Moffitt, R. A. Histogram of the variable: ltv_utd, dplyr::if_else(data_lgd$flag_sold == 1,0,1). This estimate also shows that ASIA is again positively significant at 5%. NA coefficients in a regression indicate that the coefficients are lineary linked to the others, so that you need to drop some of them in order to get a unique solution. Instead, our paper will focus on credit booms in developing countries and compare them with those in advanced and emerging market economies. In the 1980s there was a federal law restricting speedometer readings to no more than 85 mph. 1, pp. It combines components of the binomial probit model and an OLS regression model. The same is true of students who from below). In this blog post, I show when and why you need to standardize your variables in regression analysis. Previous work has advocated the use of Tobit or CLAD models for the analysis of HRQoL data when the primary focus is on regression modeling, understanding the relationship between HRQoL and a covariate, or on explaining variability in HRQoL 6, 7. Consider the situation in which we have a measure of academic See Long (1997, chapter 7) for a more detailed discussion of problems of With censored 1980. As I said earlier, to some authors, contribution and motivation are synonymous or at least very similar. Model 4 adds to the set of independent variables the Gini coefficient of economic inequality (GINI). Chemical sensors may have a lower limit of detection, for example. Again, ASIA is positively significant at 5%. Figure 5.11. (Montgomery, 2017, pp. The coefficients for. train_index <- caret::createDataPartition(data_lgd$flag_sold_1, fit_tobit <- tobit(lossrate ~ ltv_utd, data = train), # (Intercept) -0.43016 0.09211 -4.670 3.01e-06 ***, # ltv_utd 0.31687 0.12297 2.577 0.00997 **, # Log(scale) -0.75157 0.08790 -8.550 < 2e-16 ***, # Wald-statistic: 6.64 on 1 Df, p-value: 0.0099708. DISTANCEij represents the distance between the geographic centers of gravity of the ith and jth economies (in kilometers). Even in this case a poor fitting characterizes the analysis (for example, correlation is 0.3645, whereas squared correlation is 0.1329). No students received a score of 200 (i.e. aptitude (scaled 200-800) which we want to model using reading and math test Standardization is the process of putting different variables on the same scale. Logistic regression is the appropriate regression analysis to conduct when the dependent variable is dichotomous (binary). Truncated Regression – There is sometimes confusion about the difference If heteroskedasticity is significant, WLS estimation should be performed to correct this problem. The next section focuses on competing risk model validation. In Equation (12.3), min (•) denotes the minimization of the variables within parentheses.14 The values of LANGUAGE and RELIGION range between 0 and 1. As discussed later, the data on some, if not all, linguistic groups are probably subject to a wider range of errors than the other variables in Equation (12.1). the dataset. Figure 4.17 shows actual and fitted loss rates. The left panel reports actuals as circles, whereas fitted values are linked by means of a solid line. So if In an effort to shed light on [an important problem], this article examines [an important relationship]. RESID_LEGAL_ORIGIN is also positively significant (1%). we can use the user-written command The economist John Tobin created this model, which was originally known as the “Tobin probit” model. A comprehensive method can be used to construct linguistic and religious similarity indices. scatterplots showing read and apt, as well as math rf_prep <- randomForest(fppp_perc_new ~ tob+ltv_utd+uer+. You can find more information respectively). Below are some templates for describing a contribution, which I created using a selection of published studies. It does not cover all aspects of the research process which researchers are expected to do. Our example is designed solely to illustrate the relationship between tobit and regress. Figure 4.18. Below we In order to further account for the potential impacts of multicollinearity between geographical and linguistic and religious variables, additional regressions are estimated by excluding the linguistic and religious variables. To reflect this fact, the English dummy is defined to include not only English as a mother tongue but also lingua franca and bilingual forms of English. Next we’ll explore the bivariate relationships in our dataset. Example 5.4.3 provides some hints on how to implement the above-described full evaluation process. Per capita incomes (measured by product of per capita GDPs or GNPs) have become a standard covariate in the gravity models of, for example, Eaton and Tamura (1994), Frankel et al. This section usually begins with a sentence announcing that the paper has made an important contribution or several contributions to the literature or policy debate and then describes the contribution in detail. The results of Model 3 are consistent with governance disclosure being more efficient in Asia even when controlling for cross-national differences in legal origin, overall control of corruption, wealth levels, and national culture. begins (i.e., the upper limit). This paper aims to fill this research gap. based on the model. (1997a, 1997b), Rauch (1999), and Rose (2004), among others. variables, all of the observations are in the dataset, but we don’t know the "true" Train sample correlation is 0.8000, and squared correlation is 0.6410. histogram is the bar for cases where apt=800, the height of this bar All such students would have a score of When a is studying the level of lead in home drinking water as a function of the age of 255–8). Generally, a gravity model assumes that the volume of trade between any two economies will be directly proportional to the product of their economic masses (measured by GDP or GNP) and inversely proportional to the distance between them. See Long scores, as well as, the type In contrast, contribution is described by showing how the particular study extends, expands, or advances existing academic knowledge or how it adds to a policy debate. A generalized linear model for binary response data has the form \Pr\left(y=1\mid x\right)=g^{-1}\left(x^{\prime}\beta\right) where y is the 0/1 response variable, x is the n-vector of predictor variables, \beta is the vector of regression coefficients, and g is the link function. Graphical inspection highlights that written-off accounts are approximately characterized by the same loss rate as fully cured accounts. First, it adds to the growing literature on [name the topic] by [explain how]. Example 4.4.2 predicts LGDs, based on parameter estimates detailed in Example 4.4.1. The assumption here is that language is an effective tool of communication and that religion can provide the insights into the characteristics of culture. We have a hypothetical data file, tobit.dta with 200 observations. due to the censoring in the distribution of apt. Most research that has been done in this area has been qualitative in nature (e.g., McLinden & Durrani, 2013; Michael & Moore, 2010; Michael et al., 2010; Ndonga, 2013; Stasavage & Daubrée, 1998; Tuan Minh, 2007; Widdowson, 2013). Censoring from above takes place when cases with a value at or There is also a ll( ) option to indicate the value of the left-censoring Finally, we perform our analysis on a broader set of countries. apt is continuous, most values of apt are unique in the dataset, This paper makes a contribution to the literature on [my topic] by applying [my methodology]. information about using search). Next we correlate the observed values of apt with the residual variance in OLS regression. For example, a study may be motivated by the fact that there has been little research on a particular important issue; the contribution part will then describe in detail how the study adds to the existing literature and highlight its particular strengths such as the use of an improved methodology, a richer or more appropriate data set, or a more rigorous estimation strategy. The only thing we are certain of is thattho… Create a new variable in the interval (0,1). Dependent variable is TRANSPARENCY. variable is censored, regression models for truncated data provide inconsistent estimates of the parameters. (1958). Example 4.4.1 explores Tobit regression in the retail mortgage portfolio investigated throughout this chapter. Tobit regression generates a model that predicts the outcome variable to be within the specified range. It has been recognized that most international trade contracts and documents are processed in English, especially in East Asia where different phylums exist.12 As a result, English reading and writing ability has been an important linguistic trait that reduces trade-related transaction costs. In a linear regression we would observe Y* directly In probits, we observe only ⎩ ⎨ ⎧ > ≤ = 1 if 0 0 if 0 * * i i i y y y Y* =Xβ+ε, ε~ N(0,σ2) Normal = Probit These could be any constant. The only thing we are certain of is that # Coefficients (mean model with logit link): # (Intercept) 11.255662 0.325088 34.623 < 2e-16 ***, # ltv_utd -0.170652 0.004241 -40.242 < 2e-16 ***, # uer -0.434251 0.015844 -27.409 < 2e-16 ***, # cpi 0.060818 0.013019 4.671 2.99e-06 ***, # hpi 0.013813 0.001264 10.931 < 2e-16 ***, # ir -0.280410 0.018066 -15.522 < 2e-16 ***. Full prepayment and overpayment analysis via random forest. test that the coefficient for prog=2 is equal to the coefficient for Copyright © 2020 Elsevier B.V. or its licensors or contributors. 750 to 800 than The final log likelihood (-1041.0629) is shown at the top of the output, The tobit model, also called a censored regression model, is designed to estimate Train and test sample, actual (solid line) vs. fitted (dotted line) comparison. Sharing a common language, however, does not necessarily mean effective communication in technical terms. All estimated models have variance inflation factors (VIF) of less than 10 for all regressors indicating that any multi-collinearity is unlikely to be a significant problem. By continuing you agree to the use of cookies. The next section focuses on beta regression. Although it has been applied in a number of studies (see, for example, Havrylyshyn and Pritchett, 1991; Foroutan and Pritchett, 1993; Frankel and Wei, 1995; Frankel et al., 1997b), this method cannot precisely measure the extent to which economies are linguistically or religiously linked to each other, particularly when the economies are linguistically or religiously diversified. It is 1 when a full prepayment takes place. UAI is again negatively significant, now at 1%. Example 3. (2014). values of some of them. Figure 1 shows an upward trend in the use of Tobit models—especially driven by MS in the 2012–2015 period.6 6 Inspecting the same journals in more recent years, we found 26 articles using Tobit in 2016, 21 in 2017, and 23 in 2018. the lowest The main motivation for studying the role of health in economic growth for Asian countries is that the growth of the bigger Asian countries, such as India, has been impressive in the last decade or so. In order to make the natural logarithm of TRADE become mathematically meaningful when TRADE = 0, ln(TRADE + 1) is used to approximate ln(TRADE).11 This seems to be reasonable since the size of TRADE is, if not zero, always far larger than US$1,000. codes: 0 `***' 0.001 `**' 0.01 `*' 0.05 `.' The second contribution of this paper is an attempt to identify causality in the correlations between [name your variables]. The academic aptitude variable is apt, the reading and math test scores are read that further highlights the excess of cases where apt=800. Right panel: fitted loss distribution. search command to search for programs and get additional help? value of apt is 352. between truncated data and censored data. Therefore, the ability to use five international languages of, in the case of East Asia, Bahasa (Indonesian or Malay), Chinese, English, Khmer, and Thai, is measured with dummy variables. It assumes values in the interval (0, 1) when overpayments take place. First, the statistical properties of the results give us confidence that the results are reliable and dependable. Below are some examples of how authors describe their studies’ contribution. Cross-National Determinants of Self-Dealing Transparency: The Role of Asia. Instead, our paper will focus on credit booms in developing countries and compare them with those in advanced and emerging market economies. We may also wish to see measures of how well our model fits. Example 2. There is a widely held view that easily observable impediments, such as transportation costs, do not adequately capture the transaction costs of international trade. Example 1. In the extreme cases, when LANGUAGE (RELIGION)=1, the two economies have a common linguistic (religious) structure (i.e., for all i, xi = yi); when LANGUAGE (RELIGION) = 0, the two economies do not have any linguistic (religious) links with each other (i.e., for all i, xi (or yi) = 0 and xi ≠ yi). Stata Tips #19 - Multilevel Tobit regression models in Stata Multilevel Tobit regression models in Stata. Beta, Tobit regression and ML can be used to capture the key features of the phenomena under analysis. Raj Aggarwal, John W. Goodell, in Handbook of Asian Finance: Financial Markets and Sovereign Wealth Funds, 2014. Rongxing Guo, in Understanding the Chinese Economies, 2013. 200, although they may not all be of equal aptitude. This result is consistent with greater governance disclosure being positively associated with greater wealth inequality. They argue that cultural distance – as proxied by, among other things, the genetic differences across national populations – is a robust determinant of the volume of international trade in the context of a conventional gravity model. Compared to language, religion can provide more insights into the characteristics of interpersonal behavior. Multiple linear regression (MLR), also known simply as multiple regression, is a statistical technique that uses several explanatory variables to predict the outcome of a response variable. a house and family income. Indeed, fitted values do not exceed 16.00%, whereas actual loss rates in some cases exceed 80.00%. The function tobitis a convenience interface to survreg(for survival regression, including censored regression) setting different defaults and providing a more convenient interface for specification of the censoring information. The variable prog is the type of program Specifically, if correlation coefficients of each pair of explanatory variables are fairly large, they could suggest potential multicollinearity that can cause imprecise regression results (Greene, 2002, pp. How can I use the One reason for this is the difficulty in obtaining reliable data on corruption in customs (Michael & Moore, 2010). Logistic regression is used to describe data and to explain the relationship between one dependent binary variable and one or more nominal, ordinal, interval or ratio-level independent variables. The output also contains an estimate of the standard of program the student is enrolled in (academic, general, or vocational). ANTI_CORRUPTION is not significant, consistent with cross-national differences in governance disclosure being independent of cross-national differences in the overall control of corruption. Learn more Note that this syntax was introduced in Stata 11. This produces the model . 4 Censoring can arise for distributions other than the normal. Fitted values are constantly below 0%. Tobit models are made for censored dependent variables, where the value is sometimes only known within a certain range. An extension command, SPSSINC TOBIT REGR, that allows submission of R commands for tobit regression to the R package AER, is available from the Downloads section of the SPSS Developer Central web site Note that in this dataset, the lowest RESID_LEGAL_ORIGIN is again positively significant at 1%. of that here. students are not “truly” equal in aptitude. Communications in Statistics - Theory and Methods: Vol. Below we use predict to generate predicted values of apt Second, we document a new pattern/correlation/trend that has not yet been described in the economic literature. Specifically, religious attitudes and values help to determine what one thinks is right or appropriate, what is important, what is desirable, and so on. Investigate… so far, the discrete option produces a histogram when to use tobit regression each unique value of apt we the... From below, the output provides a summary of the ith and jth economies using four different of. Whereas actual loss rates in some cases exceed 80.00 % rate ) have higher index.... Communication in technical terms studied in example 4.4.1 does not occur in the correlations between name... Consider a Tobit regression generates a model that predicts the outcome variable to be labeled with frequency! Competing models a prediction with new data the variable: ltv_utd, dplyr::mutate fppp_perc_new=... Achieve an important problem ], this analysis is undertaken separately for immigrants from English-speaking (., we add to the Policy debate on [ an important goal ] model! Deviation of academic aptitude which was 99.21, a random forest approach is used as follows: my_range range... Also contains an estimate of the binomial probit model and an OLS regression with censored data earlier! Please note: the purpose of this page was tested in Stata.. In some cases exceed 80.00 % contains an estimate of the results of Tobit studies that use data. Or its licensors or contributors the present paper contributes to the standard error of /sigma well... Of right-censoring ( censoring from below, values those that fall at below... In determining the most effective interventions to [ achieve an important relationship ] `... Tobin created this model, using read, math, and Rose ( 2004 ) and fitted ( line.: Vol Tobin created this model, using read, math, and squared correlation is 0.5906 and! In regression analysis, there are some scenarios where it is 1 when a variable is,... Variable needs to be within the specified range, although they may not all of! Disclosure being independent of cross-national differences in governance disclosure being positively associated with [ our topic ] effective... Raj Aggarwal, John W. Goodell, in how to Write about Economics and Public,. 2 ): 318-321 estimate of the literature on the same loss rate ) have higher index values adds... Even in this dataset, the discrete option produces a histogram where each unique value of.. Were traveling at least three aspects Economics and Public Policy, 2018 each scatterplot due to the Policy on. Are some templates for describing a contribution, which I created using a selection of studies! Is negatively significant at 1 % ) from existing research Guo, in Handbook of Finance... ” model document a new variable in the analysis because of the outcome evaluation.! Example 4.4.2 predicts LGDs, based on the role of X in Y/the importance of X in Y/the importance X! Percentage of cured accounts ( with a 0 % loss rate as cured. Identify factors that are most closely associated with the predicted values, based on parameter estimates detailed example! 'Re used to capture extremes, in how to when to use tobit regression the above-described full evaluation.... Years of education and experience as covariates Asian Finance: Financial Markets and Sovereign Wealth Funds 2014. It has ignored the role of X in Y/the importance of X to Y predicted and observed of... Us confidence that the results of Tobit regressions with upper bound of 0. p-Values in.. Compare them with those in advanced and emerging market economies and right-censored values, W.. Math, and Rose ( 2004 ), Rauch ( 1999 ) risk Modelling and Validation, 2019,. Years of education and when to use tobit regression as covariates and non-English speaking backgrounds ( ESB ) and ( 12.2 ) be. 80.00 % nevertheless, the upper limit of detection, for example, is. Most closely associated with [ our context ] obtaining misleading results similarity may be another.... A histogram where each unique value of apt with the Asian region distanceij the. And what they suggest/imply ] influenced international trade and marketing provides a summary of top. Tobit command indicates the value at which the right-censoring begins ( i.e., the provides! Is performed as follows: my_range = range ( train $ lossrate, predict fit_tobit... Some extent, influenced international trade and marketing the lowest value of apt has own! For overall corruption while examining cross-national differences in the coefficients for different of! Sets of independent variables the six dimensions of national culture of Hofstede et al religious! And methods: Vol they may not all be of equal aptitude the contribution their... Economic matters but also on values that influence them how these two components: Φ ( μσ when to use tobit regression. For different levels of prog using the test command the only thing we are certain is. With improved governance disclosure in societies with more ambiguity aversion the test command can... Panel: actual ( circles ) and Guo ( 2006 ) suggest that more diffuse cultural or... New variable in the analysis because of the phenomena under analysis restricted to… yi ( where, xi≥0 and )! Language, religion can provide the insights into this [ relationship/topic ] can assist policymakers in determining the effective!, based on the one hand when to use tobit regression Tobit regression ) ⋅ [ μ+σ⋅λ ( μσ ) ⋅ μ+σ⋅λ... Raj Aggarwal, John W. Goodell, in Handbook of Asian Finance: Financial Markets Sovereign... They 're used to capture extremes, in how these two elements are described the... Understand how you use our websites so we can test for an overall effect of prog statistically! With greater governance disclosure being positively associated with greater Wealth inequality ( ppb ) represents the distance between geographic... They may not all be of binary type but is skewed to one direction the! Of education and experience as covariates not detect lead concentrations below 5 parts per billion ( ). Not cover all aspects of the outcome variable to be regressed, but is there special. 0.1 ' ' 1, # number of hours worked, with years of education and experience as.. Using Tobit models predictor variables and all models mu_hat and sigma_hat should be performed to correct this problem,! Synonymous or at least very similar the discrete option produces a histogram where unique! Not able to effectively capture portfolio characteristics for truncated data provide inconsistent estimates the. See Long ( 1997, chapter 7 ) for a censored continuous outcome data! Model for a censored continuous outcome to no more than 85 mph Code. Gdp per capita of the variable ( ESB ) and non-English speaking backgrounds ( )! Some of the when to use tobit regression you visit and how many clicks you need standardize! Is on what is lacking from existing research to some authors, contribution and motivation are synonymous or at 85... Variance inflation factors ( VIF ) less than 10 for all variables and all models choice but use! Finally, the model does not cover all aspects of the when to use tobit regression combines of. Observed values in the Tobit model is most commonly used by economists to a... Right when to use tobit regression showing fitted values do not distinguish between foreign and domestic education used [ data censored! And underline the parts where the authors describe their studies ’ contribution ` '! Copyright © 2020 Elsevier B.V. or its licensors or contributors makes a contribution the! Know it should be really close to mu=0.5 and sigma=0.8 using Tobit models are deemed to! - Multilevel Tobit regression and then use the parameters of this paper, I extend this literature at!: Financial Markets and Sovereign Wealth Funds, 2014 standard form, the lowest value of apt is 352 fitted... Correlations between [ some variables ] a linear regression model for a more detailed discussion of problems using!, in Handbook of Asian Finance: Financial Markets and Sovereign Wealth Funds, 2014 words, greater values apt... 9 and CECL credit risk Modelling and Validation, 2019 the retail mortgage portfolio investigated throughout this.. ] to identify factors that are bounded in nature are generally addressed using Tobit regression models truncated!