I have a repeated measures data set and I would like to create a line graph of the mean values of four groups:
  • white males
  • black males
  • white females
  • black females
which are coded as a pair of dummy variables (female, black) Y-value is mathscore, X-value is month (1-7)

I could manually calculate the mean() of each group:
Code:
mean(testscore) if(black==0) & (female==0) & (month==1)
mean(testscore) if(black==0) & (female==0) & (month==2)
​​​​​​​mean(testscore) if(black==0) & (female==0) & (month==1)
...
​​​​​​​mean(testscore) if(black==1) & (female==1) & (month==7)
but there has to be a programmatic way of doing this more elegantly.

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