Export a SAS dataset to Stata with all variable names converted to lowercase

I use both SAS and Stata and often need to transfer data between the two. SAS is case-sensitive and Stata is not. I always prefer working with lowercase variable names in Stata. The following code is used to export a SAS dataset to Stata with all variables names converted to lowercase.

The macro I use is borrowed from Adrian’s work. Thanks Adrian.

A related post can be found here: http://kaichen.work/?p=1365.


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Clean up TRACE Enhanced dataset

WRDS provides an excellent manual (link) and SAS code (link) for cleaning up the raw TRACE Enhanced bond transaction data, primarily based on the work done by Dick‐Nielsen, Jens, How to Clean Enhanced TRACE Data (December 3, 2014). Available at SSRN: https://ssrn.com/abstract=2337908. Dick‐Nielsen also provides his SAS code for the clean-up. Several papers refer to his cleaning steps.

Both WRDS and Dick-Nielsen’s codes remove cancellations, corrections, reversals, and double counting of agency trades. Dick-Nielsen’s code provides a few more options, e.g., remove commissioned trades.

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Stata command to perform Chow test

A Chow test is simply a test of whether the coefficients estimated over one group of the data are equal to the coefficients estimated over another.

I find two useful articles from Stata’s official website:

Can you explain Chow tests?
How can I compute the Chow test statistic?

Suppose we do following regressions separately in two groups:

regress y x1 x2 if group==1 and regress y x1 x2 if group==2

Then following commands will test the equality of coefficients on x1 and x2:

ge g2=(group==2)
regress y c.x1##i.g2 c.x2##i.g2
contrast g2 g2#c.x1 g2#c.x2, overall

Stata’s official website gives an example of the output:

In this example, to test the equality of coefficients on x1 and x2, 6.06 and 2.80 are the F-stats that we are looking for.

Posted in Stata | 5 Comments

SAS macro to count the number of analysts following a firm

This macro is used to count the number of analysts who followed a specific firm. Although this is a commonly used measure in literature, prior studies often give a vague description on what they do. The question is—what does “analysts following a firm” really mean?

First, it is only meaningful to count the number at a specified date.

Second, how to define “an analyst is actually following a firm”? I use the following definition: if an analyst issued any forecast (EPS or stock price or sales, anything) within a certain window (e.g., 180 days) before the specified date, then the analyst will be counted in. This definition ensures that the analyst is “actively” following the firm.

That is why my macro requires two arguments: DATE and WINDOW. This macro is used to answer such question—at a specified date, how many analysts are actively following Firm A, B, …?


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The art of regular expression

Regular expression is a powerful tool to do text search. It is the foundation of a lot of textual analysis research, though today’s textual analysis in computer science has gone far beyond text search. Regular expression operations are programming language independent. Any modern programming language supports regular expression operations well. So, if someone tells you that PERL is the best language to do text search (or textual analysis), that is plainly wrong.

Writing regular expression is work of art! You can find building blocks of regular expression here. I create this post to gather examples of regular expression that will solve certain text search questions. I will grow this post continuously.

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Stata commands to change variable names or values of string variables to all lowercase

Stata is a case-sensitive application. Sometimes this will cause a trouble. So, we may want to change variable names or values of variables to all lowercase before we start processing data. This post gives a fast way to do this.

Change variable names to all lowercase

We need to use the command rename. Instead of renaming variables one at a time, we can rename all variables in a single command (thanks Steve):

A related post can be found here: http://kaichen.work/?p=1483.

Change values of string variables to all lowercase

ustrlower(string_variable) or strlower(string_variable) will do the trick. Instead of applying ustrlower or strlower function to string variables one by one, we can benefit from lowercasing values of all string variables in a short loop. The following loop will first check the type of a variable. If it is a string variable, then change the value of the variable to all lowercase.


Posted in Stata | 2 Comments

Common commands to deal with date in Stata

egen compdatadate=eom(fiscalmonth fiscalyear)
format compdatadate %td

To be continued …

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Stata command to order tabulation result with only top values shown

tabulate varname command is handy in Stata, but sometimes it returns a too long result, if varname contains too many unique values.

The third-party command, groups, will solve the problem by showing top values only. Please use ssc install groups to install groups. The usage of group is very similar to tabulate. Here are some examples:


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Empower “and” and “or” in IF statement in Stata

Stata is a little bit awkward when using and and or in if statement, compared to SAS. For example:

In SAS, we can write if 2001 <= fyear <= 2010. But in Stata, we usually write: if fyear >= 2001 & fyear <= 2010.

In fact, Stata provides a handy inrange function. The above if statement can be written as: if inrange(fyear, 2001, 2010).

Similarly, Stata provides another inlist function. The syntax is inlist(z, a, b, ...), which returns 1 if z = a or z = b … In if statement, it is equivalent to if z = a | z = b | ...

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Display mean and median test results in Stata

Sometimes we may want to produce the following table to compare the mean and median of two groups:

First of all, please refer to this post to see Stata commands to test equality of mean and median.

However, it is time-consuming to glean numbers from the output of these Stata commands and place them in a table. It is even more struggling that you have to repeat the tedious process every time you update your sample.

I write Stata codes to streamline the process. The codes vary between unpaired (i.e., unmatched) data and paired data.

Unpaired data

The above example is unpaired data, i.e., suspect firm-years and other firm-years are not 1-to-1 or 1-to-m matched. One usage of unpaired data is the first step of Heckman’s two-step procedure, in which two groups of observations (i.e., the group that will be selected into the second step and the group that will not be selected into the second step) are stacked vertically in the dataset. The following codes are used for unpaired data. You only need to modify the first two lines to suit your data. The codes will generate a table in Stata’s output window like this:

You can then select the output and right-click “Copy as table” and paste in Excel for a quick edit. The codes use t-test for mean and Wilcoxon rank-sum test for median.

Paired data

A typical usage of paired data is to identify a matched control group for the treatment group. For example, identify a matched firm-year for an event firm-year based on a set of characteristics (same industry, same year, similar size and book-to-market), or identify a matched firm for every event firm based on the closest propensity score (i.e., propensity score matching).

The following table is an example that compares the mean and median of two matched groups—restating firms and non-restating group. Each restating firm is matched with a non-restating firm.

Because of this matching relationship, every event firm and its control firm will be placed in the same row in the dataset. In other words, event firms and control firms are aligned horizontally. The following codes are used for paired data. You only need to modify the first two lines to suit your data. You must specify the same number of variables in the matched order in the first two lines. In other words, the first variable in the first line must be paired with the first variable in the second line, and so on. The codes will generate a table in Stata’s output window like this:

The codes use paired t-test for mean and Wilcoxon rank-sign test for median.


Posted in Stata | 12 Comments