• The different analytical laboratories gave data that were adequate to provide a good estimate of the average concentration of TP, alkalinity and chlorophyll a in a given lake over year. However, from the standpoint of describing the conditions in a lake at a given time, particularly for a research-based understanding, the comparison highlighted concerns for all three variables.\ud • Both laboratories produced erroneous data for TP in the particular year of comparison and this limits the conclusions that can be drawn.\ud • At six lakes where data from CEH’s long-term monitoring programme was available it was possible to attribute the likely incorrect value and a possible cause.\ud • For alkalinity, eight of the 72 samples (11%) analysed by the EA were judged to be incorrect. Of these eight, six appeared to arise from some sort of analytical, calculation or data-entry error, while two appeared to result from a problem during sampling, possibly linked to sample contamination or incorrect bottle labelling. However, when these dubious results were removed there was an excellent agreement in values across the two laboratories.\ud • For chlorophyll a, seven of the 72 samples (10%) analysed by the EA were judged to be incorrect. Of these seven, six appeared to arise from some sort of analytical, calculation or data entry error while one appeared to result from a problem during sampling, possibly linked to sample contamination or incorrect bottle labelling. However, when these dubious results were removed the agreement between the two analytical methods was better but less good than for alkalinity and the values obtained from by the EA were on average only 79% of the concentrations measured by CEH.\ud • More effort is required to: a) ensure data are correctly entered into the spreadsheet or database and b) check the data by regular inspection, such as plotting the data for given lakes as an ongoing time-series. Ideally, this should be done by an experienced freshwater scientist and as soon as possible to allow a re-analysis where a problem is encountered.\ud • Chlorophyll a values cannot easily be compared across laboratories using different methods.\u
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