Tuesday, December 09, 2008

Lilac Bloom

Researchers at the Climate Change Research Center at the University of New Hampshire have colloborated with the organization Clean Air-Clean planet to study indicators of climate change in the North East. One of the indicators is the start of the Lilac bloom. The hypothesis is that as climate changes and temperatures rise the Lilacs will bloom earlier and earlier. The data consists of measurements made every years since numerous sites around New England: On average, the bloom has begun 1 day earlier per decade. Now is this result "statistically significant"? To assess this, one would need a model whereby some independence can be asserted. What should NOT be done is exactly what was done: a calculation of the P-value assuming independence. Now in a given year the data are measurements at different spatially local locations: so independence among observations and across years is plain silly. Having overstated their sample size by a factor of 30, the small decrease is now quite statistically significant. They should have measured every lilac and increased the sample size by a factor of 1000!


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