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Section 1.2: Measurement

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

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  1. Section 1.2:Measurement

  2. Your Class Data (dotplots):

  3. 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? - What would create bias in this activity?

  4. 1. Validity • 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.

  5. 2. Predictive Validity • 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.

  6. 3. Errors in Measurement ? True Weight Scale stuck this morning and read a pound lower Measured Value Scale Reading True Value Random Error EXAMPLE: Bathroom Scale Scale always reads 3 pounds higher = = + + Bias + +

  7. 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! - You want small variability (very little spread for your data).

  8. 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!

  9. 6. Class Example • 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.

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