Study design

Material based on Chapter 2 Introduction to Modern Statistics

Review

  • Types of data collection
    • Experimental vs observational
  • Tidy data
  • Variables
    • Explanatory vs response variables
    • Associated vs independent
    • Type
      • Numeric: discrete vs continuous
      • Categorical: nominal vs ordinal
  • Summary statistics

Learning objectives

You should be able to:

  • identify target population for your question
  • identify individual case from target population
  • describe difference between a census and a sample
  • describe difference between sample statistic and a population parameter
  • describe anecdotal evidence
  • detail why random sampling is important
  • describe how bias can be introduced into a sample

Identify the target population for your research question

  • Who was the target population for the marketing study?

  • What was an individual case from the target population

  • Was the sample the same as the target population (census)

Difference sample statistic and population parameter

  • Population parameter is what you want to know

  • Sample statistic is your estimate of of it based on your sample

Example - E1a

American households spent an average of about $52 in 2007 on Halloween merchandise such as costumes, decorations, and candy.

  • To see if this number had changed, researchers conducted a new survey in 2008 before industry numbers were reported.

  • The survey included 1,500 households and found that average Halloween spending was $58 per household.

  • Identify the population mean and the sample mean

Your turn: E2 Sleeping in college.

  • A recent article in a college newspaper stated that college students get an average of 5.5 hours of sleep each night.

  • A student who was skeptical about this value decided to conduct a survey by randomly sampling 25 students.

    • On average, the sampled students slept 6.25 hours per night

  • Identify the population mean and the sample mean

Your turn: E8 Cats on YouTube

  • Want to estimate the percentage of videos on YouTube that are cat videos.

  • Use a random video picker to select 1000 videos.

  • You find that 2% of these videos are cat videos.

  • Determine which of the following is an observation, a variable, a sample statistic (value calculated based on the observed sample), or a population parameter.

    • Percentage of all videos on YouTube that are cat videos.
    • 2%.
    • A video in your sample.
    • Whether a video is a cat video.

Principles of experimental design

  • Control - for any differences between groups (treatment and control)

  • Randomization - sample into groups to control for confounding factors

    • What could be a confounding factor with the marketing example

  • Replication

    • Single study - replicate by collecting a large enough sample
      • Pseudo-replication - fake replication multiple email addresses one person, multiple measurements one person
    • Another study - should be able to replicate results

Observational studies

  • Prospective: data in the future identify sample and follow

  • Retrospective: data from the past

Your turn E11

  • Describe the relationship between life expectancy and percentage of internet users.

  • What type of study is this?

  • State a possible confounding variable that might explain this relationship and describe its potential effect.

Anecdotal evidence

  • A personal story or example that is not based on research or evidence

Bias in sampling

  • Non-response bias
    • People who don’t respond are different from those who do
  • Convenience sampling
    • People who are easy to reach are different from those who are not

Summary

  • Identify target population for your question
  • Identify individual case from target population
  • Describe difference between a census and a sample
  • Describe difference between sample statistic and a Population parameter
  • Describe anecdotal evidence
  • Detail why random sampling is important
  • describe how bias can be introduced into a sample

Quiz