Hi lclance ,
lclance if iolite says that the average for spot #1 is 40ppm, and there are 20 data points that are averaged to get to that value, what does iolite software do if one of those points is below LOD?
iolite will include those points in the calculation of the mean (unless they are outliers, depending on your stats methods settings). I think this is valid because to determine whether a measurement is below or above LOD, you're looking at the distribution of results to see if that distribution is clearly distinct from the distribution of baseline values. Removing those values from the calculation of the distribution would skew the result and end up with an incorrect distribution.
lclance if I have 3 spots for each element for each sample and 1 of those spots is below lod (as exported from iolite) but two are not, when I'm conducting analyses, is the average of all 3 spots only taken with the two spots that did read above lod or does the third spot have to be considered and averaged in by assigning it a value of 0?
I think whether to exclude results below LOD is a decision that is normally made subsequent to processing in iolite* and perhaps should be done on a study by study basis, depending on what you're trying to achieve (and what proportion of your results are Below LOD) and what effects removing or replacing values below LOD will have. For example, if you have many Below LOD results, and replace these results with the LOD (a common approach) this may skew your statistics towards the LOD value, and give you a non-normal distribution. Similarly, choosing not to replace the values with the LOD, and using the mean and uncertainty even though it is below the LOD has other implications. For example, a very low concentration may have an uncertainty that encompasses negative values (e.g. 1± 2 ppm) and this has other implications (in reality you can't have negative concentrations even if that is implied by your results).
*Whether you include/exclude/replace below LOD values will affect the Group Stats that iolite presents in the Summary Stats tab in the Results View, as well as the stats used in some QAQC modules.
I think this is some of what @amzipkin was referring to: it's pretty involved and does require more than a blanket approach (at least that's my understanding of it).
That said, we're happy to make any changes that the community suggests, whether it be adding options for calculating group means, or something else.
Please let us know what you think.