4.1 error distribution
following the approach of hohle and hohle (2009), the extraction data is tested for normal distribution by comparing the observed and theoretical distribution of ^hi before any other processing is applied. this is done by visually analyzing the his togram and the quantile-quantile-plot (Q-Q plot)of the error dataset. the histogram depicts the frequency of the errors within a certain predefined interval. it gives a first impression of the normality of the data distribution. in fig. 4 the entity of the ^hi error is depicted in 100 equally spaced counts from a normal distribution with mean and variance from the ^h data (red line). fig. 4 allows a comparision between the observed distribution of ^hi and the expected theoretical normal distribution with mean and standard deviation estimated from the observed data. the Q-Q plots show the