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6 Cards in this Set
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Statistics in Psychological Research
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*Consistent, dependable, large effects are easy to interpret.
*But psychological effects are often small and fragile, or arise only under certain circumstances. *Psychologists use statistics to help decide wether an observed effect is real or is due to chance. *Psychologists generally use two types of statistics: -Descriptive statistics -Inferential statistics *Why use stats? To determine if "difference" is significant. |
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Descriptive Statistics
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*Mathematical summaries of results.
*Two broad categories: -Measurements of Central Tendency: "The middle range." -Measurements of Variation (also called variance or dispersion): How similar scores are (low variance), or how different they are (high variance). |
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Central Tendency
(Descriptive Statistics) |
*Mean or Average, when normal distribution.
*Median, when distribution is skewed, (*must occur in the data set, i.e. real number*) *Mode=Mean=Median, when normal distribution. |
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Measurements of Variation
(Descriptive Statistics) |
*The range is a statement of the highest and lowest observed scores.
*The standard deviation (SD) is the average distance each score is away from the mean. |
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Probability Values and Statistical Significance
(Inferential Statistics) |
*A probability value ("p" value) is a way to estimate the likelihood that a statistic from a sample is different from that parameter in the population, due to chance alone.
For example, if X and Y are correlated at 0.70 in our sample and p<0.05, there is less than a 5% chance that X and Y are reall not correlated in the population and that our 0.70 correlation is due to sampling error alone. *A result from a sample that is unlikely to have occurred by chance is interpreted as being statistically reliable or statistically significant. **Type II error: Saying there is no effect when there is one. (preferred) **Type I error: Saying there is an effect when there is none. |
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Confidence Intervals
(Inferential Statistics) |
*Sometimes we try to infer what some "true" value is in the population (such as the mean or a correlation), based on the value we observed in our sample.
*Confidence intervals state how certain we are that the true value (population parameter) lies within a certain range. *Width of the confidence interval depends on sample size (ther larger the better) and the standard deviation (the smaller the better). *Confidence intervals are typically reported at the 90%, 95%, 99% range. |