Here are 135 random numbers between 0 and 9. Each entry in our list is equally likely to be one of the 10 digits 0 through 9.
2, 9, 7, 1, 7, 8, 9, 7, 5, 9, 1, 5, 3, 2, 3, 0, 2, 3, 3, 1, 9, 6, 0, 9, 1, 7, 0, 6, 1, 0, 0, 5, 4, 6, 5, 9, 7, 4, 7, 4, 7, 5, 1, 1, 4, 4, 7, 9, 3, 8, 5, 4, 7, 6, 6, 7, 6, 1, 5, 7, 9, 7, 2, 9, 1, 3, 1, 5, 7, 6, 6, 0, 3, 9, 1, 9, 3, 1, 1, 0, 2, 6, 5, 4, 0, 0, 7, 7, 4, 0, 8, 3, 0, 8, 4, 3, 0, 4, 8, 5, 3, 3, 9, 4, 6, 1, 9, 3, 9, 1, 1, 4, 4, 3, 9, 8, 8, 6, 4, 9, 7, 5, 3, 5, 7, 0, 7, 4, 8, 6, 7, 7, 7, 7, 0
These are actually pseudo-random numbers since I generated them using a computer algorithm, but that is not important for our purposes.
A table of random digits generated from this list would be (remove the commas):
297178975915323023319609170610054659747475114479385476676157972913157660391931102654007740830843048533946193911443988649753570748677770
A table of random digits has two properties:
The table of random digits is usually written in a different form that is easier to read. We group the digits in groups of five, and separate them by a space. The rows are numbered to make the table easy to refer to, but the row numbers and spaces haven't really changed the table of random digits we had above.
| Row | Random Digits | |||
|---|---|---|---|---|
| 1 | 29717 | 89759 | 15323 | 02331 |
| 2 | 96091 | 70610 | 05465 | 97474 |
| 3 | 75114 | 47938 | 54766 | 76157 |
| 4 | 97291 | 31576 | 60391 | 93110 |
| 5 | 26540 | 07740 | 83084 | 30485 |
| 6 | 33946 | 19391 | 14439 | 88649 | 7 | 75357 | 07486 | 77770 | ... |
This is how the table in the text was produced. Note that they probably have a different set of random digits making up their random digit table, and they start their table on row 101.
Using a simple random sampling (SRS) technique can help reduce bias in choosing a sample population.
Example
You are considering moving into a condominium, but want to ensure the social atmosphere is to your liking. You decide to talk to four tenants in the condominium which are randomly chosen, to get a feeling for the place.
Use the table of random digits given above to decide which set of people to sample, given that there are 27 apartments in the complex numbered A1 to A15, B1 to B12.
Solution To answer this question, we need to order the apartments before we begin. Since the randomization will come from the table of random digits, we can just order the apartments alphabetically and sequentially:
A1 (01), A2 (02), A3 (03), A4 (04), A5 (05), A6 (06), A7 (07), A8 (08), A9 (09), A10 (10), A11 (11), A12 (12), A13 (13), A14 (14), A15 (15), B1 (16), B2 (17), B3 (18), B4 (19), B5 (20), B6 (21), B7 (22), B8 (23), B9 (24), B10 (25), B11 (26), B12 (27).
Now, turn to the random digit table. Our labels are two digits long, so we read, starting from any column we like, the numbers as two digit labels. We ignore labels which are not in our sample (anything outside 01-27), labels which are repeated, and continue reading until we get four selections from our population.
Let's choose to start in column 5.
| Row | Random Digits | 5 | 26 54 00 77 40 83 08 43 04 85 | |
|---|---|---|---|---|
| 6 | 33 94 61 93 91 14 43 98 86 49 | |||
The tenants we should interview are therefore in apartments B11 (26), A8 (08), A4 (04), A14 (14).