1.3 Sampling and Data

Data may come from a population or from a sample.  Generally, small letters like [latex]x[/latex] or [latex]y[/latex] are used to represent data values.  Most data can be put into the one of two categories:  qualitative or quantitative.

Qualitative data are the result of categorizing or describing attributes of a population.  Qualitative data are also called categorical data.  Hair color, blood type, ethnic group, the car a person drives, and the street a person lives on are examples of qualitative data.  Qualitative data are generally described by words or letters.  For instance, hair color might be black, dark brown, light brown, blonde, gray, or red.  Blood type might be AB+, O-, or B+.  Researchers often prefer to use quantitative data over qualitative data because it lends itself more easily to mathematical analysis. For example, it does not make sense to find an average hair color or blood type.

Quantitative data are always numbers.  Quantitative data are the result of counting or measuring attributes of a population. Amount of money, pulse rate, weight, the number of people living in your town, and the number of students who take statistics are examples of quantitative data.  Quantitative data may be either discrete or continuous.

All data that are the result of counting are called quantitative discrete data.  These data take on only certain numerical values.  If you count the number of phone calls you receive for each day of the week, you might get values such as zero, one, two, or three.

All data that are the result of measuring are quantitative continuous data, assuming that we can measure accurately. Measuring angles in radians might result in such numbers as [latex]\frac{\pi}{6},\;\frac{\pi}{3},\frac{\pi}{2},\; \pi,\;\frac{3\pi}{4}[/latex] and so on.  If you and your friends carry backpacks with books in them to school, the numbers of books in the backpacks are discrete data and the weights of the backpacks are continuous data.

Sampling

Gathering information about an entire population often costs too much, is too time consuming, or is virtually impossible.  Instead, we use a sample of the population.  In order to get accurate conclusions about the population from the sample, a sample should have the same characteristics as the population it represents.  Most statisticians use various methods of random sampling in an attempt to achieve this goal.  This section will describe a few of the most common methods.

There are several different methods of random sampling.  In each form of random sampling, each member of a population initially has an equal chance of being selected for the sample.  Each method has pros and cons.  The easiest method to describe is called a simple random sample.  Any group of [latex]n[/latex] individuals is equally likely to be chosen as any other group of [latex]n[/latex] individuals if the simple random sampling technique is used.  In other words, each sample of the same size has an equal chance of being selected.  For example, suppose Lisa wants to form a four-person study group (herself and three other people) from her pre-calculus class, which has 31 members not including Lisa.  To choose a simple random sample of size three from the other members of her class, Lisa could put all 31 names in a hat, shake the hat, close her eyes, and pick out three names.  A more technological way is for Lisa to first list the last names of the members of her class together with a two-digit number, as in the following table.

Lisa can use a table of random numbers (found in many statistics books and mathematical handbooks), a calculator, or a computer to generate random numbers.  For this example, suppose Lisa chooses to generate random numbers from a calculator.  The numbers generated are as follows:

[latex]0.94360 \; \; \; \; 0.99832 \; \; \; \; 0.14669 \; \; \; \; 0.51470\; \; \; \; 0.40581 \; \; \; \; 0.73381 \; \; \; \; 0.04399[/latex]

Lisa reads two-digit groups from these random numbers until she has chosen three class members (that is, she reads 0.94360 as groups 94, 43, 36, 60).  Each random number may only contribute one class member. If she needed to, Lisa could have generated more random numbers.

The random numbers 0.94360 and 0.99832 do not contain appropriate two digit numbers.  However, the third random number, 0.14669, contains 14 (the fourth random number also contains 14), the fifth random number contains 05, and the seventh random number contains 04. The two digit number 14 corresponds to Macierz, 05 corresponds to Cunningham, and 04 corresponds to Cuarismo.  Besides herself, Lisa’s group will consist of Marcierz, Cuningham, and Cuarismo.

Besides simple random sampling, there are other forms of sampling that involve a chance process for getting the sample.  Other well-known random sampling methods are the stratified sample, the cluster sample, and the systematic sample.

To choose a stratified sample, divide the population into groups called strata, and then take a proportionate number from each stratum.  For example, you could stratify (group) your college population by department and then choose a proportionate simple random sample from each stratum (each department) to get a stratified random sample.  To choose a simple random sample from each department, number each member of the first department, number each member of the second department, and do the same for the remaining departments.  Then use simple random sampling to choose proportionate numbers from the first department and do the same for each of the remaining departments. Those numbers picked from the first department, picked from the second department, and so on represent the members who make up the stratified sample.

To choose a cluster sample, divide the population into clusters (groups), and then randomly select some of the clusters.  All the members from the selected clusters are in the cluster sample.  For example, if you randomly sample four departments from your college population, the four departments make up the cluster sample.  Divide your college faculty by department.  The departments are the clusters.  Number each department, and then choose four different numbers using simple random sampling. All members of the four departments with those numbers are the cluster sample.

To choose a systematic sample, randomly select a starting point and take every n-th piece of data from a listing of the population.  For example, suppose you have to do a phone survey.  Your phone book contains 20,000 residence listings. You must choose 400 names for the sample.  Number the population from 1 to 20,000, and then use a simple random sample to pick a number that represents the first name in the sample.  Then choose every fiftieth name thereafter until you have a total of 400 names (you might have to go back to the beginning of your phone list).  Systematic sampling is frequently chosen because it is a simple method.

A type of sampling that is non-random is convenience sampling.  Convenience sampling involves using results that are readily available.  For example, a computer software store conducts a marketing study by interviewing potential customers who happen to be in the store browsing through the available software.  Such a sample is not random because only those customers in the store on that particular day have the opportunity to be in the sample.  The results of convenience sampling may be very good in some cases and highly biased (favor certain outcomes) in others.

Sampling data should be done very carefully.  Collecting data carelessly can have devastating results.  Surveys mailed to households and then returned may be very biased because they may favor a certain group.  It is better for the person conducting the survey to select the sample respondents.

True random sampling is done with replacement.  That is, once a member is picked, that member goes back into the population and thus may be chosen more than once.  However for practical reasons, in most populations, simple random sampling is done without replacement, where a member of the population may only be chosen once.  Surveys are typically done without replacement.  Most samples are taken from large populations and the sample tends to be small in comparison to the population.  Consequently, sampling without replacement is approximately the same as sampling with replacement because the chance of picking the same individual more than once with replacement is very low.

In a college population of 10,000 people, suppose we want to pick a sample of 1,000 randomly for a survey. For any particular sample of 1,000, if we are sampling with replacement:

• the chance of picking the first person is 1,000 out of 10,000 (0.1000);
• the chance of picking a different second person for this sample is 999 out of 10,000 (0.0999);
• the chance of picking the same person again is 1 out of 10,000 (very low).

If we are sampling without replacement:

• the chance of picking the first person for any particular sample is 1000 out of 10,000 (0.1000);
• the chance of picking a different second person is 999 out of 9,999 (0.0999);
• you do not replace the first person before picking the next person.

Comparing the fractions [latex]\displaystyle{\frac{999}{10,000}}[/latex] and [latex]\displaystyle{\frac{999}{9,999}}[/latex] to four decimal places, these numbers are equivalent.  So we can see that the chance of selecting a small sample from a large population is basically the same, whether or not the sampling is done with replacement.

Sampling without replacement instead of sampling with replacement becomes a mathematical issue only when the population is small.  For example, if the population is 25 people, the sample is ten, and we are sampling with replacement for any particular sample, then the chance of picking the first person is 10 out of 25, and the chance of picking a different second person is 9 out of 25 (we replace the first person).  If we sample without replacement, then the chance of picking the first person is still 10 out of 25 but the chance of picking the second person (who is different) is 9 out of 24.  Comparing the fractions [latex]\displaystyle{\frac{9}{25}=0.36}[/latex] and [latex]\displaystyle{\frac{9}{24}=0.3750}[/latex], these numbers are not equivalent.

When we analyze data, it is important to be aware of sampling errors and non-sampling errors. The actual process of sampling causes sampling error, which is the difference between the actual population parameter and the corresponding sample statistic. For example, the sample may not be large enough.  Factors not related to the sampling process cause non-sampling errors.  For example, a defective counting device can cause a non-sampling error.  In reality, a sample will never be exactly representative of the population so there will always be some sampling error.  As a rule, the larger the sample, the smaller the sampling error.

In statistics, a sampling bias is created when a sample is collected from a population and some members of the population are not as likely to be chosen as others (remember, each member of the population should have an equally likely chance of being chosen).  When a sampling bias happens, there can be incorrect conclusions drawn about the population that is being studied. 

_{Watch this video: Statistics: Sources of Bias by  Mathispower4u [4:43] (transcript available).}

Attribution

“1.2 Data, Sampling, and Variation in Data and Sampling“ in Introductory Statistics by OpenStax is licensed under a Creative Commons Attribution 4.0 International License.