Conditional probability and independence
Distinguish the chance of an event from its chance after learning that another event occurred.
Reference · Guide · Foundation
Check what the data can support
Understand the sampling design, units and distribution assumptions.
Understand
Weighted averages and percentage changes
Combine measurements using weights that match the question and keep ratio denominators explicit.
Reference · Guide · Foundation
Normal models and confidence intervals
Quantify sampling uncertainty when a distributional model and sampling design justify it.
Reference · Guide · Advanced
Calculate
Descriptive statistics
Describe a numeric sample before choosing more advanced analysis.
Tool · Calculator + guide · Applied
Correlation and simple regression
Describe a linear relationship between paired observations.
Tool · Calculator + guide · Advanced
Survey sample size for a proportion
Plan the number of completed responses for a simple random sample.
Tool · Calculator + guide · Applied