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Goals of this activity

• Introduce students to the logic of statistical significance with a randomized experiment
• Tactile and computer-based simulation
• Continue to illustrate the six-step statistical investigation process
• Empower students to explore different statistics for comparing two groups on a quantitative variable

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Additional resources:

• Example online lab assignments
• html lab
• jotform
• Runestone

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Lab 5 – Sleep Deprivation

“Sleep is required for memory consolidation - the process by which experience or training is transformed into improvements in performance.”

Stickgold et al. study

• Subjects (n = 133) were 18 to 25 years old and gave informed consent before participating in the study (Massachusetts Mental Health Center)
• Each subject was trained in a single session lasting 60-90 minutes, and was subsequently tested in a second, identical session, 3 hours to 7 days after training.

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Visual Discrimination Tasks

Sleep Deprivation

• The 21 subjects were shown either screen (a) or (b) for 17ms, then a blank screen for various lengths of × (“interstimulus interval”), then screen (c) for 17ms.
• Then asked whether they saw T or L and whether horizontal or vertical array of diagonals

1 second blank screen

• Is there are vertical array of lines and an L or is there a horizontal array of lines and a T on the first screen?

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• The first screen will appear for 17ms.
• Followed by a blank screen for 1 s.
• Followed by a masking screen.

O.5 second blank screen

• Is there are vertical array of lines and an L or is there a horizontal array of lines and a T on the first screen?

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• The first screen will appear for 17ms.
• Followed by a blank screen for 0.5 s.
• Followed by a masking screen.

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O.1 second blank screen

• Is there are vertical array of lines and an L or is there a horizontal array of lines and a T on the first screen?

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• The first screen will appear for 17ms.
• Followed by a blank screen for 0.1 s.
• Followed by a masking screen.

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Stickgold et al. study

• Subjects were tested over a range of interstimulus intervals, and the minimum ISI required to reach a threshold accuracy on the horizontal-vertical discrimination task of 80% was determined.
• Improvement for a subject was defined as the decrease in threshold ISI at retest compared to training

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Example: Are there lingering effects to sleep deprivation?

• Stickgold, James, and Hobson (2000): “Sleep is required for memory consolidation - the process by which experience or training is transformed into improvements in performance.”
• Visual discrimination task
• 60-90 minute training session
• Group A: Sleep deprived one night, unrestricted sleep 2 nights
• Group B: Unrestricted sleep all 3 nights
• Retested after 3^rd night

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Example: Lingering sleep deprivation?

• Does sleep deprivation have harmful effects on cognitive functioning three days later?
• 21 subjects; random assignment

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• Core question of inference:
• Is such an extreme difference unlikely to occur by chance (random assignment) alone (if there were no treatment effect)?

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One approach

• Calculate test statistic, p-value from approximate sampling distribution

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Another approach: Tactile simulation

• Take 21 index cards for the 21 subjects
• Write the improvement score on each card
• Shuffle the cards and deal to the two groups at random
• Determine the difference in sample means
• Repeat many times
• How unusual is it for random assignment process alone to create a difference at least as large as 15.92ms by chance alone?

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Computer simulation

• RossmanChance applets:
• www.rossmanchance.com/applets
• Randomization Test for Quantitative Response

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• URL setting to load specific dataset

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Another approach

• Simulate randomization process many times under null model, see how often such an extreme result (difference in group means) occurs

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Practical Significance

• “Effect size”: Difference in means divided by the residual standard error
• 15.93 / 11.48 = 1.39
• Cohen has suggested that effect sizes of 0.2, 0.5, and 0.8 represent small, medium, and large effect sizes respectively
• With an effect size of 0.50, we would expect about 2/3 of the data in one population to be below the mean of the other population, large enough to be “visible with the naked eye.”

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Advantages

• You can do this from beginning of course!
• Emphasizes entire process of conducting statistical investigations to answer real research questions
• From data collection to inference in one day
• As opposed to disconnected blocks of data analysis, then data collection, then probability, then statistical inference
• Leads to deeper understanding of concepts such as statistical significance, p-value, confidence
• Very powerful, easily generalized tool
• Flexibility in choice of test statistic (e.g. medians, odds ratio)
• Generalize to more than two groups

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The BEST figure in the course