I have a large dataset of environmental chemistry data. Many results are U flagged by either the lab or validators. If I want to use these results to find average values over time I see there are several options for incorporating the U flagged results
- Substitute the limit of detection (LOD)
- Substitute the LOD/2
- Substitute the LOD/sqrt(2)
- Substitute 0
- Use the median value (provided fewer than half of the values in the data set are U flagged)
- Use a trimmed mean
- Some other methods I don't know about
I feel like there are pros and cons to each of these methods (some bias low, high, skew the variability of the dataset). I do not know what standard practice is for dealing with results below detection limits. Which of these methods (or another) is typical when dealing with chemistry data with missing values?
Edit: Just to clarify, these U flagged values are not literally missing because it's not like nothing is known about them. There is some information: they are greater than or equal to 0 but less than the limit of detection.