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📘 Lesson 1 of 11 · Statistics

📋 Data Collection

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.

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01 Key Concepts

Population 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.

Qualitative vs. Quantitative Data

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

Discrete vs. Continuous Data

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

Sampling 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.

Bias in Data Collection

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

02 Solved Examples

Example 1 A researcher surveys 200 out of a city's 500,000 residents. Identify the population and the sample.
  1. The population is every resident of the city (500,000 people).
  2. The sample is the 200 residents actually surveyed.
Answer: Population = 500,000 residents; Sample = 200 surveyed residents
Example 2 Classify 'favorite ice cream flavor' as qualitative or quantitative.
  1. This describes a category or quality, not a number.
Answer: Qualitative data
Example 3 A school wants a representative sample and divides students by grade level, then randomly samples from each grade. Name this sampling method.
  1. Dividing into groups (grades) and sampling from each group is the defining feature of this method.
Answer: Stratified sampling

03 Practice Questions

1Classify 'number of pets owned' as discrete or continuous.
Discrete
2Classify 'height in cm' as discrete or continuous.
Continuous
3What sampling method uses whoever is easiest to reach?
Convenience sampling
4Why can convenience sampling introduce bias?
It may not represent the full population, over-representing easy-to-reach groups
5Is 'car brand owned' qualitative or quantitative?
Qualitative

📄 Data Collection — Downloadable Worksheet

10 questions with a full answer key. Grab the PDF to print, or try the interactive version in your browser.

🗒 Cheat Sheet ⬇ Download PDF 🖥 Interactive Worksheet