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1 Descriptive Statistics zVariable - something that can vary or change zDependent variable - something we measure zData - a collection of measurements zStatistics - summary descriptions of data (i.e., mean, medium, range)

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2 Descriptive Statistics zUsed to describe or summarize sets of data to make them more understandable ymeasures of central tendency xmean, median, mode ymeasures of variability xrange, standard deviation ymeasures of association xcorrelation coefficient

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3 Measures of Central Tendency zWhat is the average family income above? zMean - the arithmetic average zMedian - the center score zMode - the score that occurs the most

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4 Measures of Variability zRange - the difference between the highest and lowest score in a set of data zStandard deviation - reflects the average distance between every score and the mean

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5 Correlation Coefficient zOften we measure more than one variable zGrade point and SAT score zAre they related? zCorrelation statistic is a way to find out

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6 Correlation Coefficient zMeasures whether two variables change in a related way yCan be positive (max +1.00) zNegative (min -1.00) zOr not related! (~ 0.0)

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7 Inferential Statistics zDescriptive statistics summarize a data set zWe often want to go beyond the data zIs the world at large like my sample? zAre my descriptive statistics misleading? zInferential statistics give probability that the sample is like the world at large

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8 Statistics and Probability zProbability means how likely something is zHow likely are results like mine to occur by chance? zStatistical inferences ysignificant result - reflects the real world rather than chance, with high probability (e.g., >.95 ) ynot significant - results reflect chance

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9 Measurement Errors zWhy is inference based on probability instead of certainty? zData can be misleading because of variability ylow variability yhigh variability

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10 Measurement Errors zWhy is inference based on probability instead of certainty? ylow bias yhigh bias zData can be misleading because of bias

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11 Measurement Error zVariability and bias can combine Variability & Bias VariabilityBias

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12 Sources of Bias zBiased sample - when the members of a sample differ in a systematic way from the larger population the researcher is interested in zExample yinterested in all voters ycontact by telephone ybiased sample - lower economic groups may not own telephones

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13 Sources of Bias zObserver-expectancy effect yresearcher has expectations that influence measurements zSubject-expectancy effect ysubject knows design and tries to produce expected result zBlinding yminimize expectancy by removing knowledge about experimental conditions

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14 Blinding zSingle-blind study - when subjects are kept uninformed as to the treatment they are receiving zDouble-blind study - when both subjects and experimenter are kept uninformed about aspects of the study that could lead to differential expectations

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15 Ethical Issues in Psychological Research zRight to privacy zInformed consent yuse of deception zAnimal rights yIs there justification for discomfort or harm a research procedure may produce? zAPA publishes ethical guidelines

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