Sampling 05: Systematic Sampling

Definition: Systematic sampling, also known as systematic random sampling, is a method of selecting samples from a population following a structured approach.

Scenario: Measuring blood pressure of people in line at an emergency room.
- Simple Random Sampling: Each person assigned a number (1, 2, ..., n), and random numbers are selected until the sample size is achieved.
Systematic Sampling Process:
- Assign the same numbers to individuals in line and decide to select every 4th person.
- Choose a random starting point between 1 and 4 (e.g., number 3).
- Beginning with the third person, continue to select every fourth person:
  - Selected individuals: 3, 7, 11, ...
- Ensure randomness in the starting point but follow a systematic selection afterward.

Periodicity Problem:
- Taking every nth person can lead to bias if the population has a repeating pattern.
- Example: If choosing every 5th person from a line of men and women, all selected could end up being men due to arrangement.

Sampling by House Numbers:
- Two sides of a street with different socioeconomic statuses (rich vs. poor neighborhoods):
  - If start at an even-numbered house and proceed to select every second or fourth house, systematic bias can occur, overlooking entire sections of the population.

TSA Sampling:
- In security lines, TSA agents may use systematic sampling by checking every 5th or 6th person for compliance (e.g., checking for bomb residue).
- Randomly pick a starting individual ensures every person has a chance to be selected.

Systematic sampling can be efficient but must be executed carefully to avoid biases stemming from periodic sequences within the population.