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# Section 1.2: Measurement - PowerPoint PPT Presentation

Section 1.2: Measurement. Your Class Data ( dotplots ):. Questioning the Class Data:. Are the measurements valid ? - What would make them NOT valid? Are the measurements reliable ? - What would make them NOT reliable? Is there any bias in your data ?

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### Section 1.2:Measurement

• Are the measurements valid?

- What would make them NOT valid?

• Are the measurements reliable?

- What would make them NOT reliable?

• Is there any bias in your data?

- What would create bias in this activity?

• A variable is a VALID measure of a property if it is relevant or appropriate as a representation of that property.

• Sometimes it is better to use a rate than a count.

- Percentages of population versus number of people in population.

• Example: Your study would not be valid if you measured the height of students in an attempt to study students’ math grades.

• How accurate does one variable predict another?

• Does a variable predict success to a given task?

• Example: Scores from IQ Tests are used to predict intelligence.

?

True Weight

Scale stuck this morning and read a pound lower

Measured Value

True

Value

Random Error

EXAMPLE: Bathroom Scale

Scale always reads 3 pounds higher

=

=

+

+

Bias

+

+

4. Types of Errors

• Bias: When a measurement process systematically overstates or understates the true value.

- Example: When measuring everyone’s height, all rulers started at 1 inch, not 0 inches.

• Reliability: When there is a small random error after repeated measurements.

- A study is reliable if measurements are consistent!

5. Improving Reliability (reducing error)

• Find the most accurate method of measuring

• Repeat Measurements (obtain a larger sample)

- If possible, take the averages of measurement data to draw conclusions from!

• You take your blood pressure at home using a home monitor. You get the following results: 120/80, 132/90, 125/85, 110/70, 135/85. You go to the doctor and find that your actual blood pressure is 121/80.

-Is your blood pressure monitor a valid method for measuring your blood pressure? Explain.

- Does your BP monitor have a problem with reliability or bias? Explain.