Quantitative research
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Quantitative Research. Experimental. Experimental Research. Cause and effect relationships are established by manipulating the INDEPENDENT variable(s) and observing the effect on the DEPENDENT variable.

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Quantitative Research

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Quantitative research

Quantitative Research

Experimental


Experimental research

Experimental Research

  • Cause and effect relationships are established by manipulating the INDEPENDENT variable(s) and observing the effect on the DEPENDENTvariable.

  • Research design must control for the possible effects of extraneous variables that could mask, enhance, or in some way alter the effect of the independent variable on the dependent variable.


Example

Example:

General study description: Recruited obese participants will spend 3 weeks in a tightly controlled laboratory setting

Dependent Variable: Weight Loss

Independent variable: food intake

Independent variable: exercise regimen


Internal external validity

Internal & External Validity

  • Internal Validity: determined by the degree to which the observed effects of the independent variable (IV) are REAL and not caused by extraneous factors

  • Alternative explanations for the effect of the independent variable (IV) on the dependent variable (DV) threaten internal validity

  • KEY: controlling for the possible effects of extraneous variables


Internal external validity1

Internal & External Validity

  • External Validity: determined by the ability to generalize the study results beyond the study sample


Threats to internal validity

Threats to Internal Validity

alternate

explanations

  • History

  • Maturation(children)

  • Testing

  • Instrumentation

  • Selection bias

  • Mortality/attrition

  • Hawthorne

  • Placebo

    • blind vs. double blind

  • Implementation

    • fidelity


Control strategies threats to internal validity

Control StrategiesThreats to Internal Validity

  • Randomly select participants from a well-defined study population

  • Randomly assign selected participants to groups

  • Include non-treatment control groups in the research design


Final point on int ext validity

Final Point on Int/Ext Validity

  • External validity can not exist without internal validity

  • If the results of the study are not internally valid, there is nothing to generalize.

  • Researchers should be always be concerned about ensuring internal validity first.


Choosing a design

Choosing a Design

  • Identify and use a design that…

    • Controls as many extraneous variable as possible

    • Will still be practical and feasible to implement


Experimental designs

Experimental Designs

  • X =independent variable (the treatment)

  • X2 or Y = additional treatments

  • O = measurement of the dependent variable (an observation)

    • Each observation or measurement is numbered indicating order (O1, O2, O3 )

  • R = random assignment

  • Hawthorne effect


Examples of types of randomization

Examples of Types of Randomization

(Jacobsen, 2012, figure 13-6)


Non experimental designs

Non-experimental Designs

  • Survey research designs

    • Cross –sectional

    • Longitudinal

      • Trend studies –track population changes over time

        • Youth Risk Behavior Survey (YRBS) http://www.cdc.gov/HealthyYouth/yrbs/pdf/us_injury_trend_yrbs.pdf

      • Cohort study – follow a particular group or subgroup over time

        • National Longitudinal Study of Adolescent Health (Add Health) http://www.cpc.unc.edu/projects/addhealth/design

      • Panel study – examine the same group of people over time at the individual level

        • Panel Study of American Religion and Ethnicity (PS-ARE) http://www.ps-are.org/index.asp


Framework for a cohort study

Framework for a Cohort Study

(Jacobsen, 2012, figure 12-2)


Non experimental designs1

Non-experimental Designs

  • Correlational study

    • Identifies relationships and the degree or closeness of those relationships

    • A correlation exits if, when one variable increases another variable either increases or decreases in a somewhat predictable way.

    • What is the relationship between participation in intramural sports and BMI among WOU students?

    • What is the relationship between religiosity and age of sexual initiation in seventh grade students?


Types of relationships

Types of Relationships

  • Linear relationships

    • Positive: both variables move in the same direction (one variable increases as the other increases)

    • Negative: one variable moves in the opposite direction of the other (one variable increases while the other decreases)

  • Curvilinear relationships


Assessing correlation

Assessing correlation

  • Rough measure = scatter plot

  • Statistic = correlation coefficient or r (describes a sample of paired values from two different variables)

    • Measures the closeness with which the pairs of values fit a straight line

  • Range of values for r = +1.0 to -1.0

  • When r = 0, there is no correlation

  • 1.0 = perfect correlation


Interpreting a scatter plot

Interpreting a Scatter Plot

  • Line of best fit

  • http://staff.argyll.epsb.ca/jreed/math9/strand4/scatterPlot.htm


Relationships cause effect

Relationships cause & effect

  • Correlation of ice cream sales and death by drowning (r = +.86)

  • In the months when ice cream sales go up, so do deaths by drowning and likewise when ice cream sales go down, so do deaths by drowning

  • A.) Does ice cream consumption cause drowning deaths to increase? or B.) Do drowning deaths cause surviving family members and friends to eat more ice cream?


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