Re: Entering '<1' and 'BDL'

Rich,
I was thinking of something like a UV absorbance of a sample too dilute to accurately measure. You would get a reading of 0.04 or something, but since your spectrophotometer is only accurate between say, 0.4 - 3.0, it's essentially noise. Nonetheless, I can tell that by any value below my detection limit, so the value, though not accurate, would distinguish between not measured and measured but below the detection limit.

Similarly, I should think, if you measured something beyond, or off-scale, you could tell that from the value entered too. So for the spectrophotometer example, below 0.4 is noise and above 3.0 is off-scale. So you don't know what their accurate values are, you know they were "too low" to detect, or "too high" to measure with your instrument. But you know something. You then need to filter the records that can't be used for quantitative analysis. But retain them when calculating work done or other qualitative stuff.

That would allow you to use your current schema. Otherwise you need to complicate your schema, don't you?

Tom
kellert(at)ohsu(dot)edu<mailto:kellert(at)ohsu(dot)edu>
503-494-2442

On Apr 20, 2010, at 1:25 PM, Rich Shepard wrote:

On Tue, 20 Apr 2010, Tom Keller wrote:

Why distinguish "below detection limit" at the data level? I would use the
... value measured and enforce the ramification of the detection limit in
subsequent analysis. Then use NULL for missing values only. or isn't that
feasible?

Tom,

There is no measured value when it falls below the detection limit of the
instrument. Consider: you have a 6' measuring tape and you use that to
measure the length of your car. The car extends beyond the length of the
tape. What value do you enter for car length?

There are actually three situations that may occur with permit compliance
monitoring, and each must be recorded as distinct from the others. One, the
sample was not taken, or analyzed, or entered in the system. Two, the sample
was taken and analyzed, but the value falls outside the measuring range of
the instrument (or analytical method) used. Three, the sample could not be
taken because the stream channel was dry (ergo, no water to sample), the
pond frozen (ditto), or the machinery shut down and the plant not operating.
The latter two are expected and reasonable occurrences; the first one is a
no-no.

Rich

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