I would appreciate some guidance on a sample size question.
I work for an agency which collects
statistical crime data from local police agencies. These agencies serve
cities/towns which vary in size, and therefore the total number of criminal
'incidents' which they report in a year also vary.
I am periodically audited by a higher
authority. This higher authority selects about 10 agencies each time they
come to visit. They have determined that if they audit 300 incidents they
will have gotten a 'statistically valid' sample of cases for my state.
They then divide these n = 300 into
roughly equal number of cases to audit for each of the ten agencies, therefore
each agency has about 30 cases examined.
However, as stated above, the 10 agencies
serve different sized populations and report different numbers of incidents per
year.
Here are the results of the most recent
audit:
|
1st
group sampled |
|||||
|
Agency |
Records
Reviewed |
Incidents
in 2009 |
Errors
Found |
Calculated Error
Rate |
%
of All Offenses Reviewed |
|
A |
55 |
26941 |
1 |
1.8% |
0.20% |
|
B |
40 |
306 |
3 |
7.5% |
13.07% |
|
C |
40 |
3163 |
1 |
2.5% |
1.26% |
|
D |
32 |
1050 |
4 |
12.5% |
3.05% |
|
E |
32 |
179 |
6 |
18.75% |
17.88% |
|
Total |
199 |
31639 |
15 |
7.5% |
0.63% |
|
2nd
group sampled |
|||||
|
Agency |
Records
Reviewed |
Incidents
in 2009 |
Errors
Found |
Calculated Error
Rate |
%
of all incidents reviewed |
|
F |
32 |
200 |
6 |
18.75% |
16.0% |
|
G |
25 |
174 |
2 |
8.0% |
14.4% |
|
H |
25 |
9217 |
3 |
12.0% |
0.3% |
|
I |
31 |
1301 |
4 |
12.9% |
2.4% |
|
J |
27 |
2673 |
1 |
3.7% |
1.0% |
|
Total |
140 |
13565 |
33 |
23.57% |
1.0% |
My question to you all: is the
methodology specified above reasonable? What questions or issues does this
methodology suggest – if it is not reasonable.
It seems to me that this techniques
oversamples cases in agencies with few cases, and undersamples cases in larger
agencies.
| Free forum by Nabble | Edit this page |