Thursday, June 21, 2012

Not "scientifically valid"

Sounds like some of the folks at the Douglas County (Colorado) School District could use a statistics refresher course. Writes the Denver Post:
The Douglas County School District has deemed its spring survey of parents "inconclusive" — a poll in which a majority of responding parents saw the district's suspended voucher program as "unfavorable" and expressed unhappiness with the district's overall direction.
District officials contend that not enough parents participated in the survey to make it a valid representation. As a result, they stamped every page of survey results posted on the district's website in red letters saying: "Inconclusive due to insufficient response rate."
More than 4,900 parents completed the survey. There are approximately 76,500 parents whose children attend Douglas County schools. 
[...]
District spokesman Randy Barber said the approximately 6 percent of parents who responded to the survey this spring was significantly under the 30 percent the district wanted to make it "scientifically valid."
Yes, people usually have more confidence in a survey if it has a higher response rate. But that doesn't mean that the results of this survey aren't valid, and there's nothing magic or "scientific" about the 30 percent threshold. We can conduct very accurate national surveys based on just 1,000 responses -- roughly a fifth of what they managed in this parent survey. More important than overall numbers is representativeness. That is, does the sample of parents who responded to the survey look roughly like the overall population within the school district? This can probably be figured out using demographic questions (although the only one I can find in the survey is just a race question. Area of residence, income, family size, ideology, etc., would be really helpful here.) 


Of course, maybe the reason that the district rejected the results as "inconclusive" has less to do with sample size than it does with the results, which were less than flattering for the district. If they really want to know what district parents think, they could conduct a survey of a representative (and probably smaller) sample and get solid results. But if they just want certain results, then by all means they should just keep doing surveys until they get the answers they want.

2 comments:

Joe said...

It doesn't help that there is no methodology section in the report. I wonder how much they were charged for this?

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