Statistics · Quick Reference

Data Collection Cheat Sheet

Statistics · Lesson 1/11

In one line: before any statistics can be calculated, data has to be gathered carefully. How data is collected directly affects how trustworthy the conclusions drawn from it will be.

Key Ideas

1Population vs. Sample. A population is every member of the group being studied. A sample is a smaller subset actually measured, used to make inferences about the population.

2Qualitative vs. Quantitative Data. Qualitative (categorical) data describes qualities, like color or gender. Quantitative data is numeric, like height or income.

3Discrete vs. Continuous Data. Discrete data takes countable values (number of siblings). Continuous data can take any value in a range (exact weight).

4Sampling Methods. Random sampling gives everyone an equal chance of selection. Stratified sampling divides the population into groups and samples from each. Convenience sampling uses whoever is easiest to reach, but can introduce bias.

5Bias in Data Collection. Systematic errors that skew results, such as a survey only reaching people with internet access, misrepresenting the true population.

Worked Examples

A researcher surveys 200 out of a city's 500,000 residents. Identify the population and the sample.

Population = 500,000 residents; Sample = 200 surveyed residents

Classify 'favorite ice cream flavor' as qualitative or quantitative.

Qualitative data

A school wants a representative sample and divides students by grade level, then randomly samples from each grade. Name this sampling method.

Stratified sampling

Practice Yourself

Classify 'number of pets owned' as discrete or continuous.

Discrete

Classify 'height in cm' as discrete or continuous.

Continuous

What sampling method uses whoever is easiest to reach?

Convenience sampling

Why can convenience sampling introduce bias?

It may not represent the full population, over-representing easy-to-reach groups