METHODOLOGY
The 29th Annual
Phi Delta Kappa/Gallup Poll
Of the Public's Attitudes
Toward the Public Schools
Illustration by Fred Bell
METHODOLOGY |
The 29th Annual Illustration by Fred Bell |
|
RESEARCH PROCEDURE
The Sample. The sample used in this survey embraced a total of 1,517 adults (18 years of age and older), including 1,017 parents of public school children. The sample of public school parents was increased to 1,017 interviews this year from the 500 interviews customarily used. The increased sample size permits analysis and reporting of findings for subgroups within the national public school parent population. A description of the sample and methodology can be found at the end of this report.
Time of Interviewing. The fieldwork for this study was conducted during the period of 3 June to 22 June 1997.
The Report. In the tables used in this report, ''Nonpublic School Parents'' includes parents of students who attend parochial schools and parents of students who attend private or independent schools.
Due allowance must be made for statistical variation, especially in the case of findings for groups consisting of relatively few respondents, e.g., nonpublic school parents.
The findings of this report apply only to the U.S. as a whole and not to individual communities. Local surveys, using the same questions, can be conducted to determine how local areas compare with the national norm.
COMPOSITION OF THE SAMPLE
Adults |
% |
No children in school |
64 |
Public school parents |
33* |
Nonpublic school parents |
7* |
| *Total exceeds 100% because some parents have children attending more than one kind of school. | |
National |
Public School Parents | |
| Gender Men Women |
|
|
| Race White Nonwhite Black Undesignated |
|
|
| Age 18-29 years 30-49 years 50 and over Undesignated |
|
|
| Education Total college College graduate College incomplete Total high school High school graduate High school incomplete Undesignated |
|
|
| Income $50,000 and over $40,000 and over $30,000 - $39,999 $20,000 - $29,999 Under $20,000 Undesignated |
|
|
| Region East Midwest South West |
|
|
| Community Size Urban Suburban Rural Undesignated |
|
|
| *Less than one-half of 1%. | ||
DESIGN OF THE SAMPLE
For the 1997 survey the Gallup Organization used its standard national telephone sample, i.e., an unclustered, directory-assisted, random-digit telephone sample, based on a proportionate stratified sampling design.
The random-digit aspect of the sample was used to avoid ''listing'' bias. Numerous studies have shown that households with unlisted telephone numbers are different in important ways from listed households. ''Unlistedness'' is due to household mobility or to customer requests to prevent publication of the telephone number.
To avoid this source of bias, a random-digit procedure designed to provide representation of both listed and unlisted (including not-yet-listed) numbers was used.
Telephone numbers for the continental United States were stratified into four regions of the country and, within each region, further stratified into three size-of-community strata.
Only working banks of telephone numbers were selected. Eliminating non-working banks from the sample increased the likelihood that any sample telephone number would be associated with a residence.
The sample of telephone numbers produced by the described method is representative of all telephone households within the continental United States.
Within each contacted household, an interview was sought with the youngest man 18 years of age or older who was at home. If no man was home, an interview was sought with the oldest woman at home. This method of respondent selection within households produced an age distribution by sex that closely approximates the age distribution by sex of the total population.
Up to three calls were made to each selected telephone number to complete an interview. The time of day and the day of the week for callbacks were varied so as to maximize the chances of finding a respondent at home. All interviews were conducted on weekends or weekday evenings in order to contact potential respondents among the working population.
The final sample was weighted so that the distribution of the sample matched current estimates derived from the U.S. Census Bureau's Current Population Survey (CPS) for the adult population living in telephone households in the continental U.S.
As has been the case in recent years in the Phi Delta Kappa/Gallup poll series, parents of public school children were oversampled in the 1997 poll. This procedure produced a large enough sample to ensure that findings reported for ''public school parents'' are statistically significant (see Research Procedure).
SAMPLING TOLERANCES
In interpreting survey results, it should be borne in mind that all sample surveys are subject to sampling error, i.e., the extent to which the results may differ from what would be obtained if the whole population surveyed had been interviewed. The size of such sampling error depends largely on the number of interviews.
The following tables may be used in estimating the sampling error of any percentage in this report. The computed allowances have taken into account the effect of the sample design upon sampling error. They may be interpreted as indicating the range (plus or minus the figure shown) within which the results of repeated samplings in the same time period could be expected to vary 95% of the time, assuming the same sampling procedure, the same interviewers, and the same questionnaire.
The first table shows how much allowance should be made for the sampling error of a percentage:
Recommended Allowance for Sampling Error of a Percentage | |||||||
In Percentage Points | |||||||
1,500 |
1,000 |
750 |
600 |
400 |
200 |
100 | |
| Percentages near 10 | 2 |
2 |
3 |
3 |
4 |
5 |
8 |
| Percentages near 20 | 3 |
3 |
4 |
4 |
5 |
7 |
10 |
| Percentages near 30 | 3 |
4 |
4 |
5 |
6 |
8 |
12 |
| Percentages near 40 | 3 |
4 |
5 |
5 |
6 |
9 |
12 |
| Percentages near 50 | 3 |
4 |
5 |
5 |
6 |
9 |
13 |
| Percentages near 60 | 3 |
4 |
5 |
5 |
6 |
9 |
12 |
| Percentages near 70 | 3 |
4 |
4 |
5 |
6 |
8 |
12 |
| Percentages near 80 | 3 |
3 |
4 |
4 |
5 |
7 |
10 |
| Percentages near 90 | 2 |
2 |
3 |
3 |
4 |
5 |
8 |
| *The chances are 95 in 100 that the sampling error is not larger than the figures shown. | |||||||
The table would be used in the following manner: Let us say that a reported percentage is 33 for a group that includes 1,000 respondents. We go to the row for ''percentages near 30'' in the table and across to the column headed ''1,000.''
The number at this point is 4, which means that the 33% obtained in the sample is subject to a sampling error of plus or minus four points. In other words, it is very probable (95 chances out of 100) that the true figure would be somewhere between 29% and 37%, with the most likely figure the 33% obtained.
In comparing survey results in two samples, such as, for example, men and women, the question arises as to how large a difference between them must be before one can be reasonably sure that it reflects a real difference. In the tables below, the number of points that must be allowed for in such comparisons is indicated. Two tables are provided. One is for percentages near 20 or 80; the other, for percentages near 50. For percentages in between, the error to be allowed for lies between those shown in the two tables.
Recommended Allowance for Sampling Error of the Difference | ||||||
TABLE A |
In Percentage Points | |||||
Size of Sample |
1,500 |
1,000 |
750 |
600 |
400 |
200 |
1,500 |
4 |
|||||
1,000 |
4 |
5 |
||||
750 |
5 |
5 |
5 |
|||
600 |
5 |
5 |
6 |
6 |
||
400 |
6 |
6 |
6 |
7 |
7 |
|
200 |
8 |
8 |
8 |
8 |
9 |
10 |
TABLE B |
Percentages near 50 | |||||
Size of Sample |
1,500 |
1,000 |
750 |
600 |
400 |
200 |
1,500 |
5 |
|||||
1,000 |
5 |
6 |
||||
750 |
6 |
6 |
7 |
|||
600 |
6 |
7 |
7 |
7 |
||
400 |
7 |
8 |
8 |
8 |
9 |
|
200 |
10 |
10 |
10 |
10 |
11 |
13 |
| *The chances are 95 in 100 that the sampling error is not larger than the figures shown. | ||||||
Here is an example of how the tables would be used: Let us say that 50% of men respond a certain way and 40% of women respond that way also, for a difference of 10 percentage points between them. Can we say with any assurance that the 10-point difference reflects a real difference between men and women on the question? Let us consider a sample that contains approximately 750 men and 750 women.
Since the percentages are near 50, we consult Table B, and, since the two samples are about 750 persons each, we look for the number in the column headed ''750,'' which is also in the row designated ''750.'' We find the number 7 here. This means that the allowance for error should be seven points and that, in concluding that the percentage among men is somewhere between three and 17 points higher than the percentage among women, we should be wrong only about 5% of the time. In other words, we can conclude with considerable confidence that a difference exists in the direction observed and that it amounts to at least three percentage points.
If, in another case, men's responses amount to 22%, say, and women's to 24%, we consult Table A, because these percentages are near 20. We look in the column headed ''750'' and see that the number is 5. Obviously, then, the two-point difference is inconclusive.
CONDUCTING YOUR OWN POLL
The Phi Delta Kappa Center for Professional Development and Services makes available PACE (Polling Attitudes of the Community on Education) materials to enable nonspecialists to conduct scientific polls of attitude and opinion on education. The PACE manual provides detailed information on constructing questionnaires, sampling, interviewing, and analyzing data. It also includes updated census figures and new material on conducting a telephone survey. The price is $55. For information about using PACE materials, write or phone Phillip Harris at Phi Delta Kappa, P.O. Box 789, Bloomington, IN 47402-0789. Ph. 800/766-1156.
HOW TO ORDER THE POLL
The minimum order for reprints of the published version of the Phi Delta Kappa/Gallup education poll is 25 copies for $10. Additional copies are 25 cents each. This price includes postage for delivery (at the library rate). Where possible, enclose a check or money order. Address your order to Phi Delta Kappa, P.O. Box 789, Bloomington, IN 47402. Ph. 800/766-1156.
If faster delivery is desired, do not include a remittance with your order. You will be billed at the above rates plus any additional cost involved in the method of delivery. Persons who wish to order the 664-page document that is the basis of this report should contact Phi Delta Kappa, P.O. Box 789, Bloomington, IN 47402. Ph. 800/766-1156. The price is $95, postage included.
Last updated: 25 August 1997
URL: http://www.pdkintl.org/kpoll97j.htm
Contact: bucheri@pdkintl.org