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Is the Porch Index of Communicative Ability an Equal Interval Scale?
Author(s) -
LINCOLN NADINA B.,
PICKERSGILL MARY J.,
VALENTINE JOHN D.
Publication year - 1981
Publication title -
international journal of language and communication disorders
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.101
H-Index - 67
eISSN - 1460-6984
pISSN - 1368-2822
DOI - 10.3109/13682828109011399
Subject(s) - pica (typography) , psychology , interval (graph theory) , weighting , scale (ratio) , rank (graph theory) , statistics , confidence interval , audiology , developmental psychology , mathematics , medicine , geography , cartography , computer science , combinatorics , world wide web , radiology
Summary The PICA multidimensional scoring system has previously been criticised for being treated as an equal interval scale without there being any evidence to justify the assumption. Two experiments were conducted to investigate whether speech therapists, experienced in using the PICA, considered the intervals between scores to be equal. Therapists were required to judge the relative magnitude of the intervals between scores, as defined by Porch. Two groups of speech therapists rated the intervals between PICA overall scores as unequal and there was considerable agreement between the groups in the rank order of the interval sizes. Three sub‐tests of the PICA were also considered and these showed agreement with each other and with the overall PICA in the rank order of interval sizes. This indicates that the PICA scoring scale is not equal interval, implying that changes across different parts of the scale are not directly comparable when assessing changes between groups of patients or within a patient. Changes across the same intervals within all subtests examined do, however, have a consistent interpretation. This suggests that it may be worth applying a weighting system to counteract the unequal intervals between scores.