To use stratification we must be able to identify the strata. In the hedgerow intercropping example we need to know over how many rows the hedge influence extends and the size of its influence. In some studies only the first row next to the hedge has shown decreased yields, while in others the interface has had an effect over six or more rows. If the strata are poorly defined,the result will be lower precision of the estimate -- how much lower depends on the variation within each stratum. Strata may be defined by: