1. Stata Review: Part II Biost/Epi 536 Discussion Section October 13, 2009

2. Indicator (Dummy) Variables Created from an existing categorical variable (e.g., bmicat ) Assigned value of 0 or 1 1, if the condition is true 0, if the condition is false ------------------------------------------------------------------- bmicat BMI (categorical) ------------------------------------------------------------------- type: numeric (float) label: bmicat_label range: [0,3] units: 1 unique values: 4 missing.: 5/60 tabulation: Freq. Numeric Label 10 0 Underweight 17 1 Normal 17 2 Overweight 11 3 Obese 5.

3. Indicator (Dummy) Variables Example: bmicat X b0 = 1underweight 0otherwise X b1 = 1normal 0otherwise X b2 = 1overweight 0otherwise X b3 = 1obese 0otherwise

4. Generating Indicator (Dummy) Variables Option 1: Use generate ( gen ) command gen underwt = (bmicat==0) if bmicat!=. gen normwt = (bmicat==1) if bmicat!=. gen overwt = (bmicat==2) if bmicat!=. gen obese = (bmicat==3) if bmicat!=.

5. Generating Indicator (Dummy) Variables Option 1: Use generate ( gen ) command. list bmicat underwt normwt overwt obese in 31/40 +-------------------------------------------------+ | bmicat underwt normwt overwt obese | |-------------------------------------------------| 31. | Normal 0 1 0 0 | 32. | Overweight 0 0 1 0 | 33. |..... | 34. | Overweight 0 0 1 0 | 35. | Underweight 1 0 0 0 | |-------------------------------------------------| 36. | Normal 0 1 0 0 | 37. | Overweight 0 0 1 0 | 38. | Obese 0 0 0 1 | 39. | Overweight 0 0 1 0 | 40. | Underweight 1 0 0 0 | +-------------------------------------------------+

6. Generating Indicator (Dummy) Variables Option 2: Use tabulate command with generate option tabulate bmicat, generate(bmigrp). tabulate bmicat, generate(bmigrp) BMI | (categorica | l) | Freq. Percent Cum. ------------+----------------------------------- Underweight | 10 18.18 18.18 Normal | 17 30.91 49.09 Overweight | 17 30.91 80.00 Obese | 11 20.00 100.00 ------------+----------------------------------- Total | 55 100.00

7. Generating Indicator (Dummy) Variables Option 2: Use tabulate command with generate option. list bmicat bmigrp1-bmigrp4 in 31/40 +-----------------------------------------------------+ | bmicat bmigrp1 bmigrp2 bmigrp3 bmigrp4 | |-----------------------------------------------------| 31. | Normal 0 1 0 0 | 32. | Overweight 0 0 1 0 | 33. |..... | 34. | Overweight 0 0 1 0 | 35. | Underweight 1 0 0 0 | |-----------------------------------------------------| 36. | Normal 0 1 0 0 | 37. | Overweight 0 0 1 0 | 38. | Obese 0 0 0 1 | 39. | Overweight 0 0 1 0 | 40. | Underweight 1 0 0 0 | +-----------------------------------------------------+

8. Graphing in Stata 10

9. Creating Histograms Stata command: hist Example: Histogram of height, by sex hist height, by(sex)

11. Creating Histograms Stata command: hist Attach value labels to variable(s) of interest Use formatting options Example revisited: Histogram of height, by sex hist height, by(sex, title(“Distribution of height by sex”) note(“”)) xtitle(“height(in)”) scheme(s1mono)

13. Creating Box Plots Stata command: graph box Example: Box plot of height, by sex graph box height, by(sex, title(Boxplots of height by sex) note(“”)) ytitle(height(in)) scheme(s1mono)

15. Creating Box Plots Stata command: graph box Now using over option Example: Box plot of height, by sex graph box height, over(sex) title(“Boxplots of height by sex”) ytitle(“height(in)”) scheme(s1mono)

17. Creating Scatter Plots Stata command: scatter Example: Scatter plot of height and weight scatter height weight

19. Creating Scatter Plots Stata command: scatter Example: Scatter plot of height and weight by sex, with lowess smoothing twoway (scatter height weight if sex==0) /// (scatter height weight if sex==1) /// (lowess height weight if sex==0) /// (lowess height weight if sex==1)

21. Creating Scatter Plots Stata command: scatter Use formatting options Example revisited: Scatter plot of height and weight by sex, with lowess smoothing twoway(scatter height weight if sex==0,ms(D)) (scatter height weight if sex==1, ms(Oh)) (lowess height weight if sex==0) (lowess height weight if sex==1),scheme(s2mono) legend(col(2)order(1 “females” 2 “males” 3 “lowess females” 4 “lowess males”)) xtitle(weight(lbs)) ytitle(height(in)) title(Height vs. weight by sex) xlab(100(25)200) ylab(50(5)80)

23. Combining Graphs Stata command: graph combine Example: Histogram and box plot of height hist height, scheme(s1mono) name(hist) graph box height, scheme(s1mono) name(box) graph combine hist box, scheme(s1mono) title(distribution of height)