Section 1 2 measurement
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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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Section 1 2 measurement

Section 1.2:Measurement


Your class data dotplots

Your Class Data (dotplots):


Questioning the class data

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?


1 validity

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.


2 predictive validity

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.


3 errors in measurement

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

+

+


4 types of errors

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).


5 improving reliability reducing error

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!


6 class example

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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