Statistics and quantitative evaluation
Levels of measurement, central tendency and spread, correlation, significance and error types, rates and constant dollars, margins of error, and discounting.
Lesson 2 of 40 · about 33 minutes
Learning objectives
- Identify a variable's level of measurement and the statistics it supports.
- Choose the right measure of central tendency and spread for a dataset, and explain what skew does to the mean.
- Interpret a normal distribution, a correlation coefficient, and R-squared.
- Explain statistical significance, the null hypothesis, and Type I and Type II errors, and match common tests to the questions they answer.
- Calculate percent change, percentage-point change, compound annual growth, and inflation-adjusted dollars.
- Read an estimate's margin of error and confidence interval correctly.
- Calculate present value, interpret a benefit-cost ratio, and know when cost-effectiveness analysis fits better.
Key concepts
Levels of measurement
What you can do with a variable depends on what kind of numbers it holds:
| Level | What it means | Planning example | Center you can report |
|---|---|---|---|
| Nominal | Categories with no order | Land use type, commute mode | Mode |
| Ordinal | Ordered categories, uneven gaps | Survey ratings, level of service grades A to F | Median or mode |
| Interval | Equal gaps, but no true zero | Temperature in degrees Fahrenheit | Mean, median, or mode |
| Ratio | Equal gaps and a true zero | Income, population, distance, travel time | Mean, median, or mode |
Averaging ordinal ratings ("the mean satisfaction score is 3.4") is common but shaky, because the steps between "satisfied" and "very satisfied" aren't necessarily equal. Reporting the share in each category, or the median, is safer. Only ratio data support statements like "twice as much."
8 more sections follow in the full lesson.
Create a free account to see the rest of this lesson
Every lesson and all five full-length practice exams are free. An account keeps your scores and progress in one place so you can see where to study next.
Just looking? The list of lessons and the sample questions are open to everyone.