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Data from Rotten Tomatoes, Newspaper Association of America Statistics: Unlocking the Power of Data Lock 5 Data from Google, Real Clear Politics Statistics: Unlocking the Power of Data Lock 5 Data from NY Law Enforcement Agency Statistics: Unlocking the Power of Data Lock 5 It’s a Common Mistake! “The invalid assumption that correlation implies cause is probably among the two or three most serious and common errors of human reasoning.” - Stephen Jay Gould
12/23/2012 5 Statistics: Unlocking the Power of Data Lock 5 Randomization How can we make sure to avoid confounding variables? RANDOMLY assign values of the explanatory variable Statistics: Unlocking the Power of Data Lock 5 Randomized Experiment In a randomized experiment the explanatory variable for each unit is determined randomly, before the response variable is measured Statistics: Unlocking the Power of Data Lock 5 Randomized Experiment The different levels of the explanatory variable are known as treatments Randomly divide the units into groups, and randomly assign a different treatment to each group If the treatments are randomly assigned, the treatment groups should all look similar Statistics: Unlocking the Power of Data Lock 5 Randomized Experiments Because the explanatory variable is randomly assigned, it is not associated with any other variables. Confounding variables are eliminated!!! Explanatory Variable Response Variable Confounding Variable RANDOMIZED EXPERIMENT Statistics: Unlocking the Power of Data Lock 5 Exercise and the Brain A study found that elderly people who walked at least a mile a day had significantly higher brain volume (gray matter related to reasoning) and significantly lower rates of Alzheimer’s and dementia compared to those who walked less The article states: “Walking about a mile a day can increase the size of your gray matter, and greatly decrease the chances of developing Alzheimer's disease or dementia in older adults, a new study suggests.” Is this conclusion valid? (a) Yes (b) No Allen, N. “ One way to ward off Alzheimer’s: Take a Hike ,” msnbc.com, 10/13/10. Observational study cannot yield causal conclusions. Statistics: Unlocking the Power of Data Lock 5 Exercise and the Brain A sample of mice were divided randomly into two groups. One group was given access to an exercise wheel, the other group was kept sedentary The brains of mice and rats that were allowed to run on wheels pulsed with vigorous, newly born neurons, and those animals then breezed through mazes and other tests of rodent IQ ” compared to the sedentary mice Is this evidence that exercise causes an increase in brain activity and IQ, at least in mice?