Why statistics?

• Why is statistical inference important?
  • 😉 “Doctor, I want to get a test for my blood iron level.”
  • 🩺 “Okay, we will have to draw all your blood to determine that.”
  • 😱 “…”
• We are constrained by time, money, and feasibility to measure every single individual in a population
• We thus draw a sample of individuals from the population to determine its property
• By chance, properties of sample are not identical to properties of population, due to sampling or measurement error
• Statistics has two purposes on inferring unknown quantity of a population using sample:
  • Estimation of parameters (point or interval)
  • Testing of hypotheses

Random sampling

• Every individual in the population must have an equal chance of being included in the sample
• Sampling of individuals must be independent from one another

Sample of convenience

• Sample that is easily available to the sampler is usually neither unbiased nor independent
• Often seen in ecological or field studies, where samples are taken based on accessibility or proximity

Volunteer bias

• Systematic bias because the properties and behaviour of the subjects affects the chance of being sampled
• Volunteers may have distinct socioeconomic statuses, health conditions, age, proactivity, causes compared with the rest of the population

Survivorship bias