George Gallup successfully predicted the winner of the U.S.

Presidential elections in 1936, 1940 and 1944 using a very small samples,

much smaller than the ones in the Literary Digest polls.

But he used quota sampling, which again was being advocated

as a certainly an alternative to convenient sampling, and in general,

a acceptable way of drawing a sample.

So in quota sampling, targets were set for

the number of respondents with specified characteristics in the hope that they

would then compose a sample that was representative of the larger population.

So the focus was on age, sex and race, and other observed characteristics.

The resulting sample, in principle, should match the larger population

in terms of these specified characteristics.

Let's look at a hypothetical example of how quota sampling works to help

give some clarity.

So, imagine that we have a population and we're interested in composing

a sample where the quotas are set on three characteristics of the population,

age, sex and race.

So in this population, half of the population is age 21 to 50.

The other half is 50 and above.

Half the population is male, half the population is female.

90% of the population is white, 10% is black.

So if those are the three variables upon which we want to compose our quotas,

we would take these shares and then use them to work out the shares

of the population that were in each combination of categories.

So, for example, if we're looking at females, white, age 21 to 50,

50% times 50% times 90% would give you 22.5%.

Or black females age 50 and above, 2.5%,

which comes out of 50% times 50% times 10%.

So we have the shares of the population, you might say,

in each cell, each combination of age, sex, and race.

These are percentages.

Now, assuming that we want a sample with 1,000 people,

we can apply these percentages to 1,000 to

figure out what our quota would be in terms of going out to compose our sample.

So, for example, 22.5% of 1,000 would

be 225 white females who are aged 21 to 50,

25 black females aged 50 and above.

So these quotas would then be given to interviewers and

they would go out into the field and start looking for people that matched

the characteristics for each set of quoats until they filled up each cell.

So, they might keep looking for white males, age 21 to 50, and once they

had 225 that they had interviewed, they would declare that cell complete.

That the quota had been satisfied and then perhaps move on.

So this leaves a lot of discretion to the interviewer and it can lead to problems.

For example, the easiest way to fill a quota might be to go to some place where

you might, for example, find a lot of white males aged 21 to 50,

perhaps outside the gates of a factory if it were the United States in the 1940s.

So perhaps you could fill your quota in a few hours, but of course, you might be

looking at a very homogeneous group of white males aged 21 to 50 if you did that.

So, this is an example of setting a quota and

then how the instructions were actually given.

So, quota sampling turned out to have problems.

So in 1948, Gallup, who'd used quota sampling with relatively small

samples to correctly predict the Presidential elections before 1948,

incorrectly predicted Dewey as the winner of 1948 Presidential

election in the United States.

You can probably guess that that prediction was wrong because you

probably never heard of Dewey.

In fact, Truman won that election.

It turned out that the sample that Gallup was using was heavily biased and

it partly reflected the problems with quota sampling.

One problem was that quota sampling left,

again, too much discretion to the interviewer.