Measurement error in heat tolerance assays

Biological measurements inherently involve some measurement error (ME), which is a major concern because measurement accuracy (how closely a measurement reproduces the true value of the attribute being measured) and statistical power steadily decrease with increasing ME. However, ME has been largely overlooked in the thermal biology literature, which can be explained by the fact that thermotolerance estimates often involve the collapse or death of the tested individuals and measurements cannot be repeated in vivo with the same specimen under identical conditions. Here we assess inter- and intra-researcher (test-retest) reliability of heat tolerance measured as knockdown time from digital recordings of Drosophila subobscura flies individually assayed in vials with a dynamic method. We provide a summary of various estimators used to compute measurement reliability (the degree to which the measurement is affected by ME) together with their statistical properties. Our results indicate that the estimation of heat knockdown time has poor reliability: intra-researcher ME=29% and inter-researcher ME=34%. This difference is attributed to lack of ‘accurateness’ (the difference in the marginal distributions of the measurements taken by the two researchers) because measurement imprecision was essentially the same in both estimates (27%). In view of our results we conclude that thermal biologists should report the reliability of thermotolerance estimates and, when necessary, adopt some straightforward guidelines suggested here to improve measurement reliability.

► Measurements error (ME) has been overlooked in heat tolerance estimates.
► We assess the reliability of heat tolerance estimates using Drosophila subobscura.
► ME can be due to lack of accurateness and/or imprecision of measurements.
► ME is an important source of noise when assessing heat tolerance.
► Reliability should be reported and eventually improved using suggested guidelines.