Homogeneity (statistics)
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- For homogeneity of variance see homoscedasticity.
In statistics, homogeneity arises in the describing the properties of a dataset, or several datasets, and relates to the validity of the often convenient assumption that the statistical properties of any one part of an overall dataset are the same as any other part. In meta-analysis, which combines the data from several studies, homogeneity measures the differences or similarities between the several studies.
Homogeneity can be studied to several degrees of complexity. For example, considerations of homoscedasticity examine how much the variability of data-values changes throughout a dataset. However, questions of homogeneity apply to all aspects od the statistical distributions, including the location parameter. Thus, a more detailed study would examine changes to the whole of the marginal distribution. An intermediate-level study might move from looking at the variability to studying changes in the skewness. In addition to these, quesions of homogeneity apply also to the joint distributions.
The concept of homogeneity can be applied in many different ways and, for certain types of statistical analysis, it is used to look for further properties that might need to be treated as varying within a dataset once some initial types of non-homogeneity have been dealt with.
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[edit] Examples
[edit] Regression
Differences in the typical values across the dataset might initially be dealt with by constructing a regression model using certain explanatory variables to relate variations in the typical value to known quantities. There should then be a later stage of anlaysis to examine whether the errors in the predictions from the regression behave in the same way across the dataset.
[edit] Time series
The initial stages in the analysis of a time series may involve plotting values against time to examime homogeneity of the series in various ways: stability across time as opposed to a trend; stability of local fluctuations over time.
[edit] Combining information across sites
In hydrology, data-series across a number of sites composed of annual values of the within-year annual maximum river-flow are analysed. A common model is that the distributions of these values are the same for all sites apart from a simple scaling factor, so that the location and scale are linked in a simple way. There can then be questions of examining thge homogeneity across sites of the distribution of the scaled values.
[edit] Combining information sources
In meteorology, weather datasets are acquired over many years of record and, as part of this, measurements at certain stations may cease occasionally while, at around the same time, measurements may start at nearby locations. There are then questions as to whether, if the records are combined to form a single longer set of records, those records can be considered homgeneous over time.
[edit] Homogeneity within populations
Simple populations surveys may start from the idea that responses will be homogeneous across the whole of a population. Assessing the homogeneity of the population would involve looking to see whether the responses of certain identifiable sub-populations differ from those of others. For example car-owners may differ from non-car-owners, or there may be differences between different age-groups.
