---
url: "https://questionstar.com/principles-of-survey-research/chapter-2/slide-074.html"
title: "Reliability and Validity"
type: "lehrbuch-slide"
chapter: 2
chapterTitle: "Survey: Measurement and Scaling"
slide: 74
---

# Reliability and Validity

*Indicates how reliably a measurement instrument measures — i.e. how consistent the results are across repeated measurements.*

Reliability

Dependability

Indicates **how reliably** a measurement instrument measures — i.e. how **consistent** the results are across repeated measurements.

No random error: X ~R ⟶ 0 ⟹ X ~O ⟶ X ~T + X ~S

The measure is **Cronbach's alpha** (0 ≤ α ≤ 1)

Values of **α ≥ 0.7** are considered acceptable

X ~O = X ~T + X ~S + XR

Validity

Accuracy

Indicates **to what extent** a measurement instrument actually measures the matter it is meant to measure.

That is: to what extent measured differences correspond to **actual** differences between the objects (quality of the measurement).

No measurement error: X ~S ⟶ 0, X ~R ⟶ 0 ⟹ X ~O ⟶ X ~T

X ~O = X ~T + XS + XR

---

*Slide 74 of 274 · Chapter 2: Survey: Measurement and Scaling · Principles of Survey Research · Dr. Paul Marx*