Using Globals (Macros)

In this exercise you will practice using a global macro to store a cutoff that is reused in several commands. Remember: in this course, locals are preferred almost always; this exercise is to help you understand how globals work and why they can be risky.

Define a global macro named lg_cut that stores the size (in hectares) above which you will consider a plot “large”. Use 1 hectare as the cutoff.

1. Create a new indicator variable large_plot that equals 1 if plot_area_GPS is strictly greater than $lg_cut and 0 otherwise. Label the variable as “= 1 if plot area > 1 ha”. How many large plots are there in the data set?

2. Use sum and if to compute the mean yield_kg on large and non-large plots. What is mean yield in each group?

3. Download the following code files and place this in the folder where you keep all your code. That should be the path that $code points to.

• 00_main
• 01_cleaning
• 02_revise
• 03_tables

Add the line do "$code/00_main.do" to your code under **## 3.3. Run 00_main.do. What cutoff does the output claim was used (in the printed header and in the variable label)? What cutoff was actually used to generate large_plot?

4. Diagnose the inconsistency by adding these lines to the end of 03_tables.do and re-running 00_master.do

   gen         large_plot_using_current_global = plot_area_GPS > $plot_cutoff_ha
   tab         large_plot large_plot_using_current_global

Do large_plot and large_plot_using_current_global match? Why or why not?

5. Fix the problem by changing the name of the global in 02_revise. Re-run 00_main. What cutoff does the output claim was used?