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Thanks, SIQ.
I have to say that I was impressed by the magnitude of the study - it says 100,000 households were studied, of which 15,000 ran their CH on a thermostat setting of 23C or above. That stands on its own as a revelation.
The researchers are somehwat caught in a bind because, to perform it as 'proper science', it would have to be done, like you decribe, under controlled circumstances, using identikit houses and health-screened subjects.
However, human factor steps in: No-one would want to live like a lab rat for upwards of 10 years, let alone the hundreds or thousands (entire families) required to accumulate some robust statistics.
The cludgy alternative is what we have presented here - real-life houses, of diverse designs with the 'constant' being taken to be the thermostat setting.
Google search throws out numerous studies correlating the rise of central heating use in developed countries with increasing obesity (1950s onwards). Were it not for the fact that this study has been running for a decade, it would be tempting that they set out specifically to counter such findings, whereas it seems that the "central heating makes you fat" concept has only recently got into circulation.
(And, if I was speculating about such things 20 years ago but saying nothing because I couldn't produce proof then, I guess others did, as well).