There is a
considerable amount of controversy over the potential adjustment
of the 2000 Census. It is therefore important to understand what the census
is attempting to do, how sampling can help to improve the accuracy of the
census and how both the census and sampling are different from polling.
are collected when somebody needs information quickly and cheaply. A good
poll is based on a scientific sample, i.e., persons are selected randomly
and interviewers are told to interview these randomly selected people and
no one else. A good poll also has a carefully written questionnaire that the
interviewer follows to the letter. One of the best-known uses of polling is
to predict the outcome of an election. Polls are also used to obtain opinions
on subjects of national interest such as whether healthcare reform is needed
or the federal government should do something about global warming. They are
also used to test reactions to certain products or marketing campaigns.
have rather small sample sizes. A survey with 1,000 respondents will typically
have a "sampling error" of "plus or minus 3 percentage points" which is an
everyday way of saying that had the survey included everybody, the likely
result would be within 3 percentage points of outcome obtained from the sample.
This is sufficient precision for most purposes, even though the 1,000 respondents
are a tiny fraction of the total population.
to sampling are not the largest source of error in a survey, however. One
very important problem that most polls face is the fact that many people simply
will not participate. For a good poll, taken by a high quality survey research
organization, the interviews are likely to contact over 1,500 potential respondents
in order to get 1,000 to be interviewed. Many others refuse even to answer
their telephone. This hypothetical survey is likely to have another 2,000
to 3,000 numbers classified as "non-contacts," meaning that the interviewer
did not get past an answering machine, or did talk to an actual person who
told the interviewer to call back later. In sum, it might take 3,000 to 4,000
residential telephone numbers to get 1,000 actual respondents. Well under
half of all potential respondents even agree to be interviewed. After the
interviews are completed, and the polltaker prepares to publish the survey
results, (s)he needs to find a way to account for the differences between
those who do and do not agree to be interviewed.
problem in a poll is respondents providing misinformation. Many respondents
never think about the questions the interviewer asks, and the questions are
not generally on their minds when the telephone rings. As a result, respondents
frequently give answers that are "off the top of their heads" and have not
been thought about very much.
the behavior that the survey is concerned about differs from the attitudes
expressed in the survey. Democrats turn into Republicans when it actually
comes time to vote - many others who state a candidate preference don't even
cast their votes. To cite another example, respondents who state great optimism
when asked about the future of the economy may become very conservative and
risk-averse when actually developing their own investment strategies. The
divide between stated attitudes and actual behavior is perhaps the greatest
problem facing survey takers these days.
is quite different from a poll. Every ten years the Census Bureau attempts
to gather information about each household in the United States. The questions
asked on the census are not opinion questions, but rather questions that pertain
to the number of people in the household and the age and sex of each member.
While every household in America should respond to the census, many do not
get forms, do not return their forms or are otherwise not correctly counted.
Because some people are omitted, or left out, of the census, while others
are erroneously counted, the Census Bureau needs a method to test the accuracy
of their data.
of sampling is intended as a quality control check on the entire census. The
Census Bureau has selected a demographically representative sample of 314,000
households1, and it hired a team of expert interviewers to determine
who should have been counted at each of the households. The Census Bureau
is using this sample in two ways - to calculate rates of omission and counting
error. Omissions are people who should have been counted, but in fact were
not. The Census Bureau believes that there were about 8 million of these in
1990. Counting errors occur in three main ways. First, some people get counted
twice at the same address, and these are called duplicates. Second, other
people get counted at a place that is not their main residence, e.g., at a
vacation home. Third, census enumerators sometimes get incorrect information
at addresses where no one answers the door. The neighbors who do supply the
information may get it wrong.
Bureau subtracts the number of counting errors from the number of omissions
to estimate the net undercount - the difference between the number of people
actually counted and the number that should have been counted. It will calculate
separate estimates of the net undercount for 64 different groups of people.
These groups are defined by variables such as race, Hispanic origin, residential
location, and how difficult the census was to take in their local neighborhoods.
manner, the Census Bureau will be able to assess its level of error when taking
the 2000 Census. In particular, it will be able to assess whether or not the
differential undercount - the difference in the net undercounts (of minorities
and non-Hispanic Whites) was as large as it has been in all the other most
recent censuses. Because this quality check has nothing to do with opinions,
preferences, or voting patterns, it should not be referred to as "polling."
Instead, it is a quality control check on the accuracy of census data collection.
As such, it follows the same logic as the quality control check of a manufacturing
process intended to calculate the percentage of items that are defective.
is a Professor of Sociology and Statistics at Temple University and a Special
Consultant to National Economic Research Associates (NERA). He has taught
undergraduate and graduate courses on statistics and survey research. His
research has investigated census taking, as well as the general topics of
survey and statistical methodologies.
sample size of 1,000 is typical for many opinion surveys. The Census Bureau
has decided to survey 300 times this number to ensure more accurate results.