What is a Measurement?

• Concept of measurement is intuitively simple Measure something two concepts

involved The thing you are measuring The measurement you produce

A Measurement is the relation between a system of labels and the property of an empirical object or event Ex, A ruler

What is a Measurement?, con’t

• Constructs and observations Problem: Sometimes the property you

wish to measure cannot be observed directly Construct: Abstract properties of

things that cannot be measured directly

Conceptual or theoretical variables

What is a Measurement?, con’t

• Variables and variability A variable is anything that can vary

Measure property in different objects under different conditions

If you the same answer, not measuring a variable

If you get different answers, you are measuring a variable

Variability is name for differences in measurements of property Sources of variability are factors

that can cause differences

What is a Measurement?, con’t

• Variables and variability, con’t When you measure a construct, the

result depends on things other than the construct you’re measuring Measurement may respond to

irrelevant values Measurement may be affected by

random factor True value + Irrelevant value +

Random Error = Measurement

What is a Measurement?, con’t

• Operational definitions Specification of exactly what steps or

operations are conducted to arrive at a particular measurement

Converging operations

Levels of Measurement

• Nominal measurements Put names into categories Don’t contain any information about

the amount of a property

• Ordinal measurements Specifies the order of items being

measurement Symbols typically numbers, but not

always

• (Equal) Interval measurements Categories ordered by amount of

property they have Can add or subtract, but cannot

multiply or divide

• Ratio measurements Interval scale with a true zero point Can add and subtract, and multiply

and divide

Levels of Measurement, con’t

Comparison of the scales

Scale Number Properties PermissibleRepresented Transformations

-----------------------------------------------------------------------Nominal Similarities and differences Any substitution that

preserves the similarities and differences between the categories

Ordinal Similarities and differences, Any change the preserves

rank order order among member

Interval Similarities and differences, Addition of a constant,rank order, magnitude of multiplication by a differences positive constant

Ratio Similarities and differences, Multiplication by a rank order, magnitude of positive constantdifferences, ratios of properties between individuals,

meaningful zero point

How Good is the Measurement?

• Reliability A measure has high reliability if it

gives the same result every time the property is measured in the same way

How to determine reliability Test – Retest reliability Split – Half reliability Alternate form reliability Interrater reliability

How Good is the Measurement?, con’t

• Validity A measure has validity when it

reflects the construct you intend it to measure

Types of validity Face validity Criterion validity

Current validity Predictive validity

Content validity Construct validity Internal validity