More Cherry-Picking from Cycle-Helmets.Com

Last month I posted a commentary regarding incorrect information on cycle-helmets.com. I attributed a problem in their analyses of cycling surveys in South Australia to a “transcribing problem”. However, the issues seem to be much more serious than that.

In the comment section of an article I authored on The Conversation, Dorothy Robinson stated

Australians generally consider cycling a healthy activity, so the discrepancy between the two sets of tables in the South Australian report might reflects a reluctance to admit they cycled less because of helmet laws. The “really big” boo-boos Linda talks about were caused by her looking at the wrong tables. Table numbers are now included in http://www.cycle-helmets.com/helmet-law-spin.html so that others will not make the mistake of attributing the differences between these tables to “transcribing errors”.

The website now refers the reader to Tables 5a and 5b (Destination of bicycle trips in the last 7 days). This isn’t completely correct as total responders were taken from Tables 1a and 1b (Frequency of bicycle riding).

The total cycling in the past week do not match up between Tables 1a/1b and 5a/5b. This is likely due to (near) complete responses for amount of cycling but missing responses for destinations. This is common when conducting surveys and highlights the problem with combining such tables, especially when there is no need to do so. In other words, if you’re really interested in comparing cycling rates before and after helmet legislation, why would you not use the frequency of cycling tables?

There is also the issue of throwing away usable data. Tables 1a and 1b contain information for four categories of cycling frequency (“At least once a week”, “At least once a month”, “At least once every 3 months”, “Less often or Never”). This information is mostly thrown out by combining the total responses for destinations in Tables 5a and 5b with the total cyclists in Tables 1a and 1b. Here is a summary of the proportions of cycling in South Australia across age groups and gender for years 1990 and 1993.

Cycling in South Australia
1990 1993
At least weekly 21.8 21.0
At least monthly 5.2 6.0
At least every 3 months 3.9 4.4
Less often or never 69.1 68.6

These results suggest the SA helmet law had no impact on the amount of cycling. The suggestion by Robinson that responders are reluctant “to admit they cycled less because of helmet laws” is unsubstantiated. If someone is reticent to admit they don’t exercise, this would apply to both the 1990 and 1993 surveys.

I’d like to be wrong about this, but the analysis on this website reeks of fishing for results that support a pre-determined conclusion.

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