Category: Uncategorized

Picking up where I left off at the end of last week’s ‘Metrics Monday post, I wanted to continue discussing the interpretation of coefficients this week.

Specifically, I wanted to discuss the interpretation of coefficients on dummy variables in semi-logarithmic equations. What’s a semi-logarithmic equation? It’s an equation of the form

[math]\ln{y} = \alpha + \beta{D} + \gamma{x} + \epsilon[/math],*

On the subject of tests used to compare two distributions, Varun writes with two questions. His first question is as follows:

I teach part of a data analysis course at our institute. With the example of auto.dta the comes with Stata, we found that the variable miles per gallon (mpg) to be not normally distributed.
To find out whether mpg is statistically different for domestic (a sub-sample of 52 cases) and foreign (a sub-sample of 22 cases) cars (total sample size of 74), I told students there are two nonparametric tests, the Wilcoxon ranks-sum test and the two-sample Kolmogorov-Smirnov test. The Wilcoxon rank-sum test tests whether medians are significantly different while the two-sample Kolmogorov-Smirnov test tests whether distributions are different both groups. My students asked me which one they should go for between these two tests for mpg in auto.dta.

It is not uncommon for the New York Times to discuss food policy. It is much less common for the newspaper of record to discuss Food Policy, the Elsevier journal I have the honor of co-editing, along with my Mario Mazzocchi. Yet the Gray Lady did just that last week when it discussed food labeling:

The Senate could soon join the House to try to make it harder for consumers to know what is in their food by prohibiting state governments from requiring the labeling of genetically modified foods. This is a bad idea that lawmakers and the Obama administration should oppose. …

There is no harm in providing consumers more information about their food. A study published in the journal Food Policy in 2014 found that labels about genetic modification did not influence what people thought about those foods. Some companies are deciding on their own to increase the information they provide to consumers without fear of losing sales.