5.2c Assign the participants to your experimental conditions

[Mike Dillinger, PhD]

Another very important step for improving external validity is to randomize which participants are assigned to which experimental conditions. Even if selection of participants is not random, you can make an effort to assign them randomly to the different experimental conditions. This will improve your ability to generalize your findings.

In practice, researchers emphasize random assignment (of participants to experimental conditions) instead of random selection because random assignment is more practical to carry out. Perhaps the most common technique for random assignment is to use a table of random numbers like the one below.

Say that you need to assign 40 participants to 4 experimental conditions. You can follow these steps:

• Start with any of your experimental conditions
• Choose a two-digit number arbitrarily (from the serial number of a dollar bill, a license plate, the page of a book opened at random, etc.).
• Use the first digit as the column number and the second digit as the row number to find where to start in your table of random numbers. Say you start with 21. Then you will begin by looking at the second column, first number: 62349.
• Go down the column, one number at a time. Look at the last one or two two digits (if your total is less than 100). For 62349, 49 is more than your total, so you can skip it or use 9 as the first number. So, participant number 9 will participate in the first experimental condition.
• The next number, going down the column, is 35050, but 50 is also more than your total. If the last two digits of the number are less than 41, then write that number on the list for experimental condition #1 until you have enough participants for condition 1. If the number is more than 40 (your total) or was already seen, then skip it or use the last digit. If you get to the bottom of a column, go to the next column.
• Repeat until you fill all of the experimental conditions with enough participants.

You end up with a list of participant numbers for each experimental condition. You can number participants by the order in which they arrive or by how they are seated. Your list will tell you which people will participate in each experimental condition.

Why is this random? For one thing, the assignment does not depend on any characteristics of the participants or of the experimenter. If you did the sampling yourself, you might unconsciously assign the smart (or male, or white, or …) participants to one condition or other. For another, the random number table is designed to provide a sequence that’s very close to pure randomness.

Read this topic next:

Develop your Materials

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Table of Random Numbers 39634 62349 74088 65564 16379 19713 39153 69459 17986 24537
14595 35050 40469 27478 44526 67331 93365 54526 22356 93208
30734 71571 83722 79712 25775 65178 07763 82928 31131 30196
64628 89126 91254 24090 25752 03091 39411 73146 06089 15630
42831 95113 43511 42082 15140 34733 68076 18292 69486 80468

80583 70361 41047 26792 78466 03395 17635 09697 82447 31405
00209 90404 99457 72570 42194 49043 24330 14939 09865 45906
05409 20830 01911 60767 55248 79253 12317 84120 77772 50103
95836 22530 91785 80210 34361 52228 33869 94332 83868 61672
65358 70469 87149 89509 72176 18103 55169 79954 72002 20582

72249 04037 36192 40221 14918 53437 60571 40995 55006 10694
41692 40581 93050 48734 34652 41577 04631 49184 39295 81776
61885 50796 96822 82002 07973 52925 75467 86013 98072 91942
48917 48129 48624 48248 91465 54898 61220 18721 67387 66575
88378 84299 12193 03785 49314 39761 99132 28775 45276 91816

77800 25734 09801 92087 02955 12872 89848 48579 06028 13827
24028 03405 01178 06316 81916 40170 53665 87202 88638 47121
86558 84750 43994 01760 96205 27937 45416 71964 52261 30781
78545 49201 05329 14182 10971 90472 44682 39304 19819 55799
14969 64623 82780 35686 30941 14622 04126 25498 95452 63937

58697 31973 06303 94202 62287 56164 79157 98375 24558 99241
38449 46438 91579 01907 72146 05764 22400 94490 49833 09258
62134 87244 73348 80114 78490 64735 31010 66975 28652 36166
72749 13347 65030 26128 49067 27904 49953 74674 94617 13317
81638 36566 42709 33717 59943 12027 46547 61303 46699 76243

46574 79670 10342 89543 75030 23428 29541 32501 89422 87474
11873 57196 32209 67663 07990 12288 59245 83638 23642 61715
13862 72778 09949 23096 01791 19472 14634 31690 36602 62943
08312 27886 82321 28666 72998 22514 51054 22940 31842 54245
11071 44430 94664 91294 35163 05494 32882 23904 41340 61185

82509 11842 86963 50307 07510 32545 90717 46856 86079 13769
07426 67341 80314 58910 93948 85738 69444 09370 58194 28207
57696 25592 91221 95386 15857 84645 89659 80535 93233 82798
08074 89810 48521 90740 02687 83117 74920 25954 99629 78978
20128 53721 01518 40699 20849 04710 38989 91322 56057 58573

00190 27157 83208 79446 92987 61357 38752 55424 94518 45205
23798 55425 32454 34611 39605 39981 74691 40836 30812 38563
85306 57995 68222 39055 43890 36956 84861 63624 04961 55439
99719 36036 74274 53901 34643 06157 89500 57514 93977 42403
95970 81452 48873 00784 58347 40269 11880 43395 28249 38743

56651 91460 92462 98566 72062 18556 55052 47614 80044 60015
71499 80220 35750 67337 47556 55272 55249 79100 34014 17037
66660 78443 47545 70736 65419 77489 70831 73237 14970 23129
35483 84563 79956 88618 54619 24853 59783 47537 88822 47227
09262 25041 57862 19203 86103 02800 23198 70639 43757 52064