I signed up to teach again this semester. I am demonstrating on a rather nice course covering social science data and methods using Stata (STAT6077, for those in Southampton: it was the course that made me realise I was doing the right MSc). Today was computer lab number one. These are the questions I answered:
Q: What does the 25th income percentile mean? A: It’s the level of income below which one quarter of the incomes are found).
Q: Which of these equivalized incomes gives the right answer? A: None of them. Annoying isn’t it? For the uninitiated, income data is often collected at a household level. Analysis, meanwhile, frequently takes place at an individual level. To bridge the gap, you have some choices. You can pretend that one person in the household gets to control the whole income (i.e. pretend that individual income equals household income). Or you can pretend that household income is shared evenly among all household members. Or you can assign 20% of the income to the kids and divide the rest evenly between the adults. Or you can do as the OECD does and treat individual income as household income divided by the square root of household size. I think this might be the time (Monday of Week One) to quote George Box: “Essentially, all models are wrong, but some are useful”.
Q: How do I access the University filestore? A: It’s on the H: drive. If you want to, for example, keep a log of your Stata workshop session, you can tell Stata:
log using h:\workshop.log, t
This will open a log file for your session and save it in your personal filestore (don’t forget to close your log at the end of the session). You can access your filestore from any computer in the university and also via a VPN connection at home.
Q: Why are there any negative household incomes in the Family Resources Survey? A: Negative incomes are usually discounted for analysis purposes (thank you Dr Google!). The variable is constructed using all income, minus council tax, pension contributions, some particular insurance payments, student loan repayments and child maintenance. There are 36 individuals (out of more than 52,000) in 17 households with a negative income. And 19 of those were children. Must be pretty grim.
Q: How does this .do file work? A: It’s magic. And you can keep a log of everything you do too. In fact, you probably should. All the details were in the introductory session on Stata in Week 0 and are available on Yves Berger’s website.
Q: Can I use Stata at home? A: Sadly not. University of Southampton only has on-site licenses. You can, however, buy a student license from Timberlake on their GradPlan. You’ll end up buying Stata 13 when the computer labs are running Stata 12, though… And it will cost a minimum of £120.
Q: What is this coefficent of variation, then? A: Instead of answering this question, allow me to introduce you to the statistical equivalent of Wikipedia. It’s great, up-to-date, and (unlike Wikipedia) completely acceptable as a reference/citation. It’s based at UCLA: http://www.ats.ucla.edu/stat/