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Is anyone familiar with a suitable transformation of time and temperature values into a single score?

I am working with microbiological data and I have three continuous factors all measured at distinct intervals: Growth Time (3 levels), Aging Time (4 levels) and Growth Temperature (3 levels).

I would like to convert the three factors into a score similar to growing degree days (GDD) which is a weather-based indicator used in crop sciences which incorporates time and temperature. However, I am struggling to find a similar example in microbiological data.

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    $\begingroup$ For home energy use during cool/cold seasons, there are also 'heating degree days'. That requires a 'ceiling' temperature above which no prudent person would turn on the heat (maybe around $68^oF = 20^oC).$ Then, as I recall, there is a (problematic?) assumption that fuel requirements for heating are linear in degrees below that upper bound. Exact details of the 'index' may have been adapted from some sort of regression with actual data. // Is there a temp below which there is little or no growth of your bacteria and above that temp does growth rate increase linearly with temp? $\endgroup$ – BruceET Oct 2 '19 at 16:53
  • $\begingroup$ Well, we see no growth at 2C, delayed onset of growth at 10C and then exponential growth to the stationary phase (equilibrium) at 25C. Then during the aging phase, things remain mostly stationary and for growth time, we will see an increase in bacteria. I was thinking of trying to convert things into some kind of ratio score but from what I have tried things get complex when I try that. $\endgroup$ – Matthew Wells Oct 2 '19 at 17:55
  • $\begingroup$ I guess it is best to analyze your data using all three of your factors. $\endgroup$ – BruceET Oct 2 '19 at 18:32

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