5. All measurements involving length have been taken at keel level, since it is only the lower part of a wreck that is generally found. The sample size is too small to be statistically significant. Nevertheless there is a central tendency in the measurements, represented in the tables by the middle number in a range of measured numbers. The implied variability is interpreted to be the result of two factors, i.e. measurement inaccuracy and variability in shipbuilding. Standardisation and industrialisation in shipbuilding occurs after the construction date of the wrecks listed in the tables, therefore no two ships are exactly the same. The term first bulkhead refers to the most forward bulkhead in a ship.