MEASUREMENT ERROR AND MORPHOMETRIC STUDIES - STATISTICAL POWER AND OBSERVER EXPERIENCE

We estimated measurement error (ME) associated with 15 skeletal characters in seven species of passerine birds. The total sample variance of the characters, from three measurements of each character on each individual, was partitioned into among- and within-individual components of variation. Measurement error, defined as that portion of the total sample variance made up by within-individual variation, ranged from <1% to >80%. Percent ME was inversely related to the size of a character, among-individual variation, and experience of the measurer. Other sources of ME included the character's flexibility and the definition of character landmarks. Although observer experience reduced ME, the effect was only substantial after more specimens were measured than are often used in morphometric studies. The magnitude of statistically detectable differences between groups was examined in relation to the level of ME and the sample sizes of groups. Measurement error increased the chances of finding no group difference, i.e., making type II errors. Type II errors can be reduced either by averaging two or more repeated measurements of the same individual, thereby reducing ME, or by increasing the sample size. We show how averaging repeated measurements changes the minimum detectable effect size between groups when both ME and sample size vary. The pattern of character-specific measurement error is consistent across passerine species; both reliable and unreliable characters for morphometric studies are identified.