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Lab IV: Outline, Part 1 Use of correlated versus independent t- tests – Sample Experiment Introduction to a web-based stats program: Vassarstats How to graph Ms and SDs, an example

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Independent and Correlated (Paired) t-tests The Terrible t’s

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I pity the fool who doesn’t know when to use correlated vs. independent t-tests!

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Sample Experiment: t-test Examples Hypothesis 1: Exposure to bright light will increase gill flare in male Betta splendens. Hypothesis 2: Male Betta splendens exposed to bright light will have longer gill flare durations than those not exposed to bright light.

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Sample Experiment: t-test Examples Subjects: 4 adult male Betta splendens (A, B, C, D) A

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Exposed fish to each other for 10 min, recorded gill flare B A C D Trial 1

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Trial 1 Gill Flare (/10 min.) ABCD 100 s120 s90 s115 s = baseline duration of gill flare

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Exposed Fish A & Fish C to bright light for 5 min. B A C D (no light)

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Exposed fish to each other for 10 min, recorded gill flare B A C D Trial 2

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Trial 2 Gill Flare (/10 min.) AB (no light) CD (no light) 200 s130 s185 s125 s = duration of gill flare after light/no light

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Hypothesis 1: Exposure to bright light will increase gill flare in male Betta splendens. FishNo Light (Before) Light (After) A100 s200 s C90 s185 s *We want to compare each fish’s score on one condition (“before exposure”) to its score on another condition (“after exposure”)

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Correlated (or Paired) t-test Scores between conditions are for same subject i.e., Fish A has a score for both “light” and “no light”, and Fish B has a score for both conditions Hence, scores are said to be “paired” or “correlated”) FishNo Light (Before) Light (After) A100 s200 s C90 s185 s

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How to use Vassarstats for t-tests http://faculty.vassar.edu/lowry/VassarStats.html http://faculty.vassar.edu/lowry/VassarStats.html t-tests and procedures

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= Number of rows 2

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Row 1 = Fish A’s scores Row 2 = Fish C’s scores Xa = before light exposure Xb = after light exposure

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Because our hypothesis was unidirectional (meaning we predicted change in a single, specific direction), we can use the one-tailed value.

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Now just copy and paste

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How to Report Results: Examples “Exposure to bright light significantly increases gill flare duration in male Betta splendens (t = -39, df = 1, p <.05).” Must also include Ms and SDs in a table or graph. “Gill flare duration after light exposure (M = 192.5, SD = 10.61) was significantly greater than before light exposure (M = 92.5, SD = 7.07); t(1) = -39, p <.05.”

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Hypothesis 2: Male Betta splendens exposed to bright light will have longer gill flare durations than those not exposed to bright light. SubjectLightSubjectNo Light A200 sB130 s C185 sD125 s *Across conditions, we are comparing the scores of two different fish; hence, the scores are independent of each other

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Independent t-test The scores between the two conditions are from different subjects, which makes them independent The scores in the “Light” condition are not correlated with scores in the “no light” condition SubjectLightSubjectNo Light A200 sB130 s C185 sD125 s

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How many scores are in each column? (If unequal, pick larger.) 2

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Xa = Light Xb = No Light

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How to Report Results: Examples “Male Betta splendens that were exposed to bright light (M = 192.5, SD = 10.61) flared their gills for longer durations than those not exposed to bright light (M = 127.5, SD = 3.54); t(2) = 8.22, p <.05.” Or, can give Ms and SDs in a table or graph.

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How to graph Ms and SDs for Duck Lab

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Mean for C1 Mean for C2 SD for C1 SD for C2 HINT: The columns correspond to those used for your t-test…

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