The standard error is the standard deviation of the sampling distribution of the statistic. For this reason standard deviation is more commonly used than variance as a measure of variability. As with variance it represents the average spread (variability) of the data but it is adjusted so that it maintains the same units of measurement as the original data. The standard deviation is the square root of the variance. This means that it is best used with ordinal or ranked data. Need to follow a particular distribution (such as the normal distribution). It departs from Pearson's r in that it does not rely on the assumptions of a parametric test to be met - that is the data does It measures the strength and direction of a relationship between two variables and ranges from -1 to 1. Spearman's rho, or Spearman's correlation coefficient, is similar to pearson's r. See the histograms below for examples! Skewed distributions can sometimes indicate floor/ceiling effects. Negative skew is when the more frequent values are at the upper end of the scale with A distribution is positively skewed when the more frequent values are clustered at the lower end of the scale and the 'tail' of the distribution points towards the upper end. Skew is used to refer to the symmetry of a normal distribution. Simple linear regression models can be represented as 'lines of best fit' or regression lines on scatterplots (shown below). Y = outcome variable, X = explanatory variable, b 0 = value of outcome when explanatory variable has a value of zero (intercept), b 1 = regression coefficient for the explanatory variable, ε i = error.The formula for a straight line is the basis for this model: This is commonly called a 3D scatterplot.Ī simple linear model in which the value of one variable can be used to predict the value of an outcome variable. Scatterplots can be build for three variables if a Z-axis is included along with the X and Y axis. A variable is assigned to each axis of the graph and each 'dot' represents one participant. This is non-systematic error - the effect is random, assuming of course that the sample was chosen at random from the population.Ī scatterplot is a graph that plots the values of one variable against the corresponding values of another. It is the distribution of the possible values we could get when drawing a sample of a given size from the population.Įssentially this is the error caused by observing a sample rather than the entire population of interest. If we drew enough samples our values could be plotted as a frequency distribution the mean) we would find that these values differed somewhat due to random variation between the samples ( sampling error).
If we were to take multiple samples of a given size from a population and then calculate a statistic for each sample (e.g. Samples can be drawn in a number of ways but must be representative of the population to achieve reliable and valid results (random selection of units is the best way to achieve a representative sample).
Rebranded PASW (Predictive Analytics SoftWare), presumably just to confuse everyone, but is now referred to as IBM SPSS.Ī sample is a selection of cases drawn from a population and used to make inferences about that population. It is the most widely used statistical package in the social science. It allows users to perform a vast number of statistical analyses and includes a substantial tool set for regression analyses. SPSS (Statistical Package for the Social Sciences) is a computer program designed to organize and analyse quantitative data.
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