Class 23: Sperm Count Revisited


Remember the article "Declining sperm counts" from the British Medical Journal. (We have extra copies if you need one.)

The section subtitled "Results" begins with the sentence "When the age of donors was examined with respect to the year of their first donation, there was no evidence that the age of donors had changed during the period of data collection." What do you think this means, and why do they mention it?

In Table 1, Laurie was surprised by the size of the standard deviations for sperm concentrations. Does this point to a flaw in the data? How do you explain it?

Earlier on this page, under "Statistical Analysis", the article says "The distribution of variables was examined and when appropriate, variables were normalised before analysis by log transformation." What does this mean, and why do they do it?

Back under "Results", the article mentions that sperm concentration decreased by 2.1% per year, with similar decreases for total number of sperm and total number of motile sperm. Where do these figures come from? If the trend continues, when is Doomsday?

What questions do you have about this article?

Journal question

Record in your journals your final thoughts about this particular study and your thoughts about Doomsday