math

Discrete variable

A variable that is counted and takes distinct values.

Continuous variable

A variable that is measured and can take any value within a range.

How can you tell discrete from continuous?

Counted = discrete; measured = continuous.

Reaction time rounded to the nearest 5 ms

Continuous, because time is measured.

Number of bird calls

Discrete, because calls are counted.

Volume of a bird call (dB)

Continuous, because volume is measured.

Number of defects per batch

Discrete, because defects are counted.

Weight of defective items

Continuous, because weight is measured.

Stratified sampling

Divide the population into groups, then randomly sample from each group.

Systematic sampling

Select people at fixed intervals, such as every 12th person.

Convenience sampling

Choose people who are easiest to reach.

Quota sampling

Select people until a set number from each group is reached, without requiring random selection.

Key difference between stratified and quota sampling

Stratified uses random selection; quota does not require random selection.

Researcher randomly selects students from each IB subject group

Stratified sampling.

Theme park interviews every 12th visitor

Systematic sampling.

Student surveys classmates they meet in the hallway

Convenience sampling.

Company interviews 50 people in each age group until filled

Quota sampling.

One disadvantage of convenience sampling

It may be biased and unrepresentative.

Why use stratified sampling?

To ensure important population groups are represented.

Clue that identifies systematic sampling

A fixed interval, such as every 5th or 12th person.