| Case study 11 - Brachiopod directional statistics |
Data file: directions.dat
The orientations (OBHL, 0-180 degrees) and facing directions (FDAC, 0-360 degrees) of the hinge lines of 94 specimens of the Ordovician brachiopod Eochonetes, taken from the same bedding plane, are found in the file directions.dat. Open the file, and select the 'FDAC' column (PAST does not have any functions for analysing orientation data). Then select 'Directions' in the 'Geom' menu. Tick the 'Geo convention' (0 degrees upwards, clockwise direction), and the 'Equal area' boxes:
By looking at the rose diagram, do the directions seem randomly oriented? Two statistical tests for directional data are given. The first is the Rayleigh test, which gives the probability that the data are not unidirectional. The other is a chi-square test, giving the probability that the directions are totally evenly spread.
What is your conclusion? Are the hinge lines randomly oriented, or do they show a unidirectional or a bidirectional distribution?
More information about directional analysis can be found in the manual.