Download Stata introduction, Exercises

Stata introduction, Exercises
Hein Stigum
Data files for the course are found on the net. This Stata command will set the web address:
webuse set “http://www.med.uio.no/forskning/doktorgradkarriere/forskerutdanning/kurs/biostatistikk/mf9510-logistisk-regresjon-overlevelsesanalyse-cox/»
This Stata command will open the datafile:
webuse "birth1.dta", clear
1. Start Stata, Open the data file ”birth1.dta”. Run the command “describe” from the command
window. Use the menu to do a summary on birth weight (weight) (first item in all submenus to
Statistics).
2. Open a new do-file. Copy the commands you have used so far to the do file. Type the command
“tab sex”, mark and run it from the do-file.
3. Find the mean of mother’s age (Hint: mean mage). Advanced question: Suppose you do not
trust a normality assumption for your data. How can you estimate the “se” of the mean? (hint:
run “help mean” and look for vce and bootstrap. vce=variance-covariance estimation)
4. Do a summary of mother’s age (mage). Do a summary of mother’s age with extra details. Do a
summary of mother’s age for each sex.
5. Summarize mother’s age if gestational age is greater that 260 (if gest>260), how many
subjects? Rerun the command excluding missing in gestational age (summarize mage if
gest>260 & gest<. ), how many subjects now? Why is there a difference in the number of
subjects!!! (Hint: missing).
6. Make a plot of the birth weight distribution (Hint: kdensity).
7. The variable “magegr2” contains mother’s age in two groups. Do “tab magegr2” and “tab
magegr2, nolab” to find the groups and the coding. An alternative to find coding is to list all
labels: “label list”, try! Make a plot of the birth weight distribution for each of the two groups of
mother’s age.
8. Test if the mean birth weights are different in the two groups of mother’s age. (Hint ttest).
9. Make a scatter plot of birth weight versus mother’s age (birth weight on the y-axis). Does birth
weight increase with mother’s age? Redo the plot with a linear fit line (Hint: (lfit y x). Is the
relation really linear? Redo the plot with a fractional polynomial fit line with confidence
intervals (Hint: add (fpfitci y x) as the second of the 3 plot parts. Redo the plot with a title and
an xtitle and an ytitle.
10. Make a table of selected statistics (Number of cases, some percentiles and min and max) of
birth weight for each of the two groups of mother’s age. (Hint: tabstat)
Last 30 minutes: Plenary discussion of the exercises