Examples
The examples in this section give you an idea of how smbase works in different situations. The first specification is:
a;smbase=30;flush
tab prefer region;op=1
tab prefer region;op=2
l region
col 115;Base;North;South;East;West
l prefer
col 120;Base;Brand A;Brand B;Brand C;Brand D;Brand E
This creates two similar tables, one showing absolutes only and the other showing column percentages only. Percentages in the second table are suppressed if the base is less than 30.
Absolutes
Base North South East West
Base 170 27 54 40 49
Brand A 38 6 11 8 13
Brand B 57 8 24 13 12
Brand C 29 5 8 5 11
Brand D 34 7 9 8 10
Brand E 11 1 2 5 3 |
Col percents
Base North South East West
Base 170 27** 54 40 49
Brand A 22.4% 20.4% 20.0% 26.5%
Brand B 33.5% 44.4% 32.5% 24.5%
Brand C 17.1% 14.8% 12.5% 22.4%
Brand D 20.0% 16.7% 20.0% 20.4%
Brand E 6.5% 3.7% 12.5% 6.1%
** very small base (under 30): percents suppressed |
The first table is flagged with op=1 only so the specification of smbase=30 on the a statement is ignored for this table. The second table has the same rows and columns but shows column percentages only. When Quantum processes the specification for this table it takes each cell in turn and compares the column base for that cell with 30. If the base is less than 30, Quantum suppresses all percentages for that cell. This is how the column percentages for North were suppressed.
The asterisks next to the base of 27 indicate that it is a small base; the footnote explains this. Notice that it reports the value specified with smbase=.
The next specification has a mean element and a No answer element that does not contribute to the base:
a;smbase=40;flush;nopc
tab rating grid;op=2
l rating
n10Base
n01Brand A;col(a)=131
n01Brand B;col(a)=132
n01Brand C;col(a)=133
n01Brand D;col(a)=134
side
col a00;Base;Excellent;%fac=5-1;Very Good;Good;Satisfactory;Poor
n01No Answer;c=ca00n'1/5';nofac
n12Mean;dec=2
The table it creates is:
Col percents
Base Brand A Brand B Brand C Brand D
Base 181 34** 58 40 49
Excellent 21.0 19.0 20.0 26.5
Very Good 31.5 41.4 32.5 24.5
Good 16.0 13.8 12.5 22.4
Satisfactory 18.8 15.5 20.0 20.4
Poor 12.2 10.3 12.5 6.1
No Answer 0.6 0.0 2.5 0.0
Mean 3.31 ** 3.43 ** 3.45
** very small base (under 40): percent suppressed |
The main point to notice in this table is the difference between the columns for Brands A and C. The mean is suppressed in both columns and the cells are flagged with two asterisks indicating that suppression is because the numbers of respondents contributing to the means (in statistical terms, the sums-of-n) are less than 40.
Looking at the base for Brand C you might not expect any percentages to be suppressed because the base is not less than the smbase value, but since the percentage for No Answer in this row is greater than zero you can assume that there is at least one person excluded from the mean. The sum-of-n is therefore less than the required value so the mean is suppressed.
The column for Brand A has all the rating percentages suppressed because the base is less than 40. The mean is suppressed because the number of people contributing to it is less than 40.
The next specification requests a T-test on column proportions:
a;smbase=40;flush
tab brand region;op=12
l brand
col 112;Base;Brand A;Brand B;Brand C;Brand D
l region
tstat prop;elms=ABCD;clevel=90
n10Base
n01North;c=c111'1';id=A
n01South;c=c111'2';id=B
n01East;c=c111'3';id=C
n01West;c=c111'4';id=D
The table is:
Absolutes/col percents
Base North South East West
(A) (B) (C) (D)
Base 265 101 64* 72* 28**
Brand A 43 6 24 13 0
16.2% 5.9% 37.5%AC 18.1%A
Brand B 67 22 16 17 12
25.3% 21.8% 25.0% 23.6%
Brand C 111 59 24 17 11
41.9% 58.5%BC 37.5%C 23.6%
Brand D 44 14 0 25 5
16.6% 13.9%B 0.0% 34.7%AB
----------------------------------------
Proportions: Columns Tested (10% risk level) - A/B/C/D
* small base; ** Very small base (under 30) ineligible for sig testing |
The point to notice in this table is that the T statistic footnote about very small bases has overridden the one you would normally see showing small bases for percentages.
Here is the same table run with the small base for T statistics set to 25. Notice the different footnote and the fact that column D is now included in the tests even though the percentages are suppressed:
Absolutes/col percents
Base North South East West
(A) (B) (C) (D)
Base 265 101 64* 72* 28**
Brand A 43 6 24 13 0
16.2% 5.9% 37.5%AC 18.1%AD
Brand B 67 22 16 17 12
25.3% 21.8% 25.0% 23.6%ABC
Brand C 111 59 24 17 11
41.9% 58.5%BCD 37.5%C 23.6%
Brand D 44 14 0 25 5
16.6% 13.9%B 0.0% 34.7%AB B
----------------------------------------
Proportions: Columns Tested (10% risk level) - A/B/C/D
* small base
** Very small base (under 30) ineligible for sig testing |
See also