33171 - Modeling change Kristin Sainani Ph.D....

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1 Modeling change Kristin Sainani Ph.D. http://www.stanford.edu/~kcobb Stanford University Department of Health Research  and Policy
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2 Within vs. Between subject  effects… Solution for Fixed Effects Standard Effect Estimate Error DF t Value Pr > |t| Intercept 38.1287 4.1727 5 9.14 0.0003 time -0.08163 0.3234 16 -0.25 0.8039 chem -0.01283 0.003125 16 -4.11 0.0008 Example 1:  last week’s example. CHEM: -.01283 A significant chemical effect (time-dependent predictor) could either represent a between-subjects effect or a within-subjects effect. Because these data were made-up, we happened to know that there was primarily a within-subjects effect…
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3 Example 1. .from last time… id time1 time2 time3 time4 chem1 chem2 chem3 chem4 1 20 18 15 20 1000 1100 1200 1300 2 22 24 18 22 1000 1000 1005 950 3 14 10 24 10 1000 1999 800 1700 4 38 34 32 34 1000 1100 1150 1100 5 25 29 25 29 1000 1000 1050 1010 6 30 28 26 14 1000 1100 1109 1500 6 patients with depression are given a drug that increases levels of a “happy  chemical” in the brain.  At baseline, all 6 patients have similar levels of this happy  chemical and scores >=14 on a depression scale. Researchers measure  depression score and brain-chemical levels at three subsequent time points: at 2  months, 3 months, and 6 months post-baseline. Here are the data in broad form:
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4 Example 2… Same as example 1, but made up to  have more between-subjects effect than  within-subjects effect. id time1 time2 time3 time4 chem1 chem2 chem3 chem4 1 50 48 45 40 200 350 400 500 2 22 24 18 22 900 920 805 950 3 44 40 34 40 500 459 880 500 4 38 34 32 34 700 770 950 800 5 15 19 15 19 1000 1000 1050 1010 6 50 58 56 54 100 110 210 220
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5 Example 3… Same as example 1, but made up to  have ONLY between-subjects effect id time1 time2 time3 time4 chem1 chem2 chem3 chem4 1 51 48 45 50 510 520 510 525 2 47 43 46 45 710 720 690 680 3 42 42 41 38 910 890 900 905 4 31 32 31 32 1105 1110 1109 1090 5 26 27 26 28 1505 1505 1510 1590 6 19 20 21 18 1750 1600 1680 1740
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Example 2 (more between-subjects effects):  showing two subjects… id=1: score id=2: chem id=1: chem id=2: score
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Example 2:
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Example 2:
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Example 2:
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Example 2:
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Example 2:
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Example 2:
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Example  3: all between-subjects effects
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Example 3:
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Example 3:
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Example 3:
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Example 3:
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Example 3:
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Results, example 1… Solution for Fixed Effects Standard Effect Estimate Error DF t Value Pr > |t| Intercept 38.1287 4.1727 5 9.14 0.0003 chem -0.01283 0.003125 16 -4.11 0.0008 time -0.08163 0.3234 16 -0.25 0.8039 Example 1:  significant chem effect. CHEM: -.01283
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33171 - Modeling change Kristin Sainani Ph.D....

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