# PDF Applied Regression Analysis Using Stata

By default, coefplot retrieves the point estimates from the first equation in vector e b and computes confidence intervals from the variance estimates found in matrix e V. See the Estimates and Confidence intervals examples for information on how to change these defaults. Furthermore, coefplot can also read results from matrices that are not stored as part of an estimation set; see Plotting results from matrices below. By default, coefplot uses a horizontal layout in which the names of the coefficients are placed on the Y-axis and the estimates and their confidence intervals are plotted along the X-axis.

Specify option vertical to use a vertical layout:. Note that, because the axes were flipped, we now have to use yline 0 instead of xline 0. By default, coefplot displays all coefficients from the first equation of a model. Alternatively, options keep and drop can be used to specify the elements to be displayed. Furthermore, coefplot automatically excluded coefficients that are flagged as "omitted" or as "base levels". To include such coefficients in the plot, specify options omitted and baselevels.

For example, if you want to display all equations from a multinomial logit model including the equation for the base outcome for which all coefficients are zero by definition , type:. For detailed information on the syntax, see the description of the keep option in the help file. Here is a further example that illustrates how keep can be used to select different coefficients depending on equation:. These options specify the information to be collected, affect the rendition of the series, and provide a label for the series in the legend. A basic example is as follows:. To specify separate options for an individual model, enclose the model and its options in parentheses.

For example, to add a label for each plot in the legend, to use alternative plot styles, and to change the marker symbol, you could type:.

Option msymbol is specified as a global option so that the same symbol is used in both series. To use different symbols, include an individual msymbol option for each model. Alternatively, you can also use p1 , p2 , etc. To deactivate the automatic offsets, you can specify global option nooffsets. Alternatively, custom offsets may be specified by the offset option if offset is specified for at least one model, automatic offsets are disabled. The spacing between coefficients is one unit, so usually offsets between —0. For example, if you want to use smaller offsets than the default, you could type:.

If the dependent variables of the models you want to include in the graph have different scales, it can be useful to employ the axis plot option to assign specific axes to the models. For example, to include a regression on price and a regression on weight in the same graph, type:. For example, if you want to draw a graph comparing bivariate and multivariate effects, you could type:.

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When merging multiple models you may need to apply some renaming of coefficients, because coefficients that have the same name will be printed on top of each other. This can be achieved by applying the rename option to the individual models. An alternative approach is presented in Model names as coefficient names. An example with one model per subgraph is as follows:. Option byopts xrescale has been added so that the two subgraphs can have different scales.

Furthermore, the plot labels for the legend were set within the first subgraph. They could also have been specified within the second subgraph, as plot styles are recycled with each new subgraph and plot options are collected across subgraphs unless norecycle is specified; see below. If the subgraphs do not contain the same number of models, it may be necessary to insert "empty" models to achieve the correct alignment. As evident in the last example, coefplot recycles plot styles within each subgraph.

## Applied Regression Analysis Using STATA - opinagsepenf.ml Pages 51 - 73 - Text Version | FlipHTML5

If you want each subgraph to use its own set of styles, apply the norecycle option:. Use option byopts byopts to determine how subgraphs are combined. For example, to use a compact style and stack the subgraphs in one column, you could type:. Note that Stata renders the titles of the subgraphs as "subtitles". Hence, you can use the subtitle option to change their styling:.

Sometimes it makes sense to arrange coefficients in separate subgraphs with individual scales, as the size of coefficients may vary considerably. For example, when comparing results by subgroups or estimation techniques, the focus usually lies on differences across models and less on differences within models, so that it appears natural to use individuals subgraphs for the different coefficients. Creating subgraphs by coefficients requires lengthy commands as for each coefficient a separate piece of subgraph syntax has to be put together.

Most people, however, do not understand what a change in the logit means. Odds are also not easy to understand, nevertheless this is the standard interpretation in the literature. The Odds CDU vs. Others is in the East smaller by the factor 0. Note: Odds are difficult to understand.

This leads to often erroneous interpretations. Example 2: e. For every year the odds increase by 2. No, because e. Probability interpretation This is the most natural interpretation, since most people have an intuitive understanding of what a probability is. The drawback is, however, that these effects depend on the X-value see plot above.

Example 1: The discrete effect is.

Example 2: Mean age is Note: The linear probability model coefficients are identical with these effects! Marginal effects Stata computes marginal probability effects. These are easier to compute, but they are only approximations to the discrete effects. There are different criteria to do this. The best known is maximum likelihood ML. ML estimates have some desirable statistical properties asymptotic. One has to solve them by iterative numerical algorithms.

With one degree of freedom we can reject the H 0. Use also the LR-test to test restrictions on a set of coefficients. Model fit With nonmetric Y we no longer can define a unique measure of fit like R 2 this is due to the different conceptions of variation in nonmetric models. Instead there are many pseudo-R 2 measures.

This measure is suggested by the authors of several simulation studies, because it most closely approximates the R 2 obtained from regressions on the underlying latent variable. A completely different approach has been suggested by Raftery see Long, pp. He favors the use of the Bayesian information criterion BIC. This measure can also be used to compare non-nested models! Stata drops this variable automatically other programs do not! Functional form Use scattergram with lowess see above. Influential data We investigate not single cases but X-patterns. There are K patterns, m k is the number of cases with pattern k.

The saturated model fits the data perfectly see example 1. Using Pearson residuals we can construct measures of influence. If one lists these patterns one can see that these are young woman who vote for CDU. The reason might be the nonlinearity at young ages that we observed earlier.

The practical disadvantage is that it is hard to calculate probabilities by hand. We can apply all procedures from above analogously only the odds interpretation does not work.

### What is Stata?

Since logistic and normal distribution are very similar, results are in most situations identical for all practical purposes. Coefficients can be transformed by a scaling factor multiply probit coefficients by 1. Only in the tails results may be different. Estimation is done by ML. This, however, is not true as can be seen from the crosstab.

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## Applied Regression Analysis Using STATA

P0 This is similar to the binary model and not very helpful. They are not easy to understand, but they do not depend on the values of X. Example 1: The odds effect for SPD is e. It is common to evaluate this formula at the mean of X possibly dummies set to 0 or 1. Further, it becomes clear that the sign of the marginal effect can be different from the sign of the logit coefficient. It might even be the case that the marginal effect changes sign while X changes! Clearly, we should compute them at different X-values, or even better, produce conditional effect plots.

Stata computes marginal effects. Stata has also an ado by Scott Long that computes discrete effects. Thus, it is better to compute these. However, keep in mind that the discrete effects also depend on the X-value.