(5) Wild readings. All data should be recorded. None of it should
be rejected at the time of the reading.
However, when a wild reading is
encountered, the meteorologist has a choice of three alternatives:
(a) Rejection. You can assume it to be a gross error and reject
If desired, another reading can be made to take its place.
should be established for rejection.
For example, when a value has a
probability of occurrence of less than 1/20, it is to be rejected. In doing
this he is assuming (with a 20 to 1 gamble) that it is a gross error rather
You can assume it to be valid but with a low
The proper treatment here is weighting, not
giving it the strength of the other readings.
(c) Routine procedure. You can ignore that it is a wild value and
treat it the same as the other values. Of course, you are taking a chance.
If it were a gross error (mistake), it would damage the value of the
average. If it were a valid random error, who knows but what it is correct
and the other values are random extremes.
g. Precision and accuracy.
(1) The dictionary defines these words each in terms of the other,
indicating that they mean about the same thing. In metrology, we need to
make finer distinctions. Precision has meaning only when applied to a group
of measurements, never to a single measurement.
It refers only to the
reproducibility of the results of measurement. The smaller the spread, the
greater the precision.
Accuracy refers to the degree to which the
measurement indicates the truth. It can pertain to a single measurement or
the average of a group of measurements. Figure 4 represents the results of
firing 10 shots each at four different targets. If a good quality gun were
clamped in a vise but little effort were made to aim it, the score would be
as in A, figure 4. If it were a poor gun but well aimed, we might get a
score such as B, figure 4. It may be noted that precision depends upon the
absence of random errors within the gun itself, whereas accuracy is a
function of systematic errors outside of the gun (poor aim, wind, bullet
A person firing the gun from his shoulder could introduce
errors of both types.