INT J LANG COMMUN DISORD, MARCH–APRIL 2014, VOL. 49, NO. 2, 215–227 Research Report Stability of language performance at 4 and 5 years: measurement and participant variability Patricia Eadie† , Cattram Nguyen‡§, John Carlin‡§, Edith Bavin¶, Lesley Bretherton‖ and Sheena Reilly§# ∗∗ †Department of Audiology & Speech Pathology, University of Melbourne, Carlton, VIC, Australia ‡Clinical Epidemiology & Biostatistics Unit, Murdoch Childrens Research Institute, Melbourne, VIC, Australia §School of Population & Global Health, University of Melbourne, Melbourne, VIC, Australia ¶School of Psychological Science, La Trobe University, Melbourne, VIC, Australia ‖Psychology Service, Royal Children’s Hospital, Melbourne, VIC, Australia #Hearing Language & Literacy, Murdoch Children’s Research Institute, Melbourne, VIC, Australia ∗∗ Speech Pathology Department, Royal Children’s Hospital, Melbourne, VIC, Australia (Received March 2013; accepted September 2013) Abstract Background: Language impairment (LI) in the preschool years is known to vary over time. Stability in the diagnosis of LI may be influenced by children’s individual variability, the measurement error of commonly used assessment instruments and the cut-points used to define impairment. Aims: To investigate the agreement between two different age-based versions of a language assessment instrument and the stability of the classification of LI using the two measures over a 12-month period. Methods & Procedures: A total of 945 participants completed the Clinical Evaluation of Language Fundamentals (CELF—Preschool 2 or 4th Edn) at 4 and 5 years of age. Agreement and stability were analysed using Bland– Altman plots, correlation and odds ratios. Sensitivity and specificity were calculated for two thresholds of the CELF-P2 using the diagnostic category on the child’s subsequent CELF-4. Outcomes & Results: For all CELF scores, mean differences for the cohort between 4 and 5 years were within 1.5 scale score units. In contrast, at the individual level variability was found across the range of scores and was of a greater magnitude than previously reported. Stability in LI classification was low, with 36% of 5-year-olds with LI (defined as a standard score below –1.25) classified as typical at 4 years, even though odds ratios calculated from classifications at the two time points suggested that 4-year-olds with LI had 23 times greater odds than their typical peers to receive a diagnosis of LI at 5 years. The CELF-P2 did not demonstrate adequate levels of diagnostic accuracy for LI at 5 years: sensitivity of 64% and specificity of 92.9%. Conclusions: Substantial variability across the entire range of possible CELF scores was observed in this community cohort between the ages of 4 and 5 years. The stability of LI classification was lower than that reported in previous research conducted primarily on smaller clinical cohorts. The current study’s results suggest that the variability observed in developmental language pathways is the result of a combination of limitations in measurement instruments, individual children’s abilities and the arbitrary nature of the boundaries defining LI. Keywords: language, measurement, stability, reliability. What this paper adds? There are limited research data available to inform speech and language therapists about both the diagnostic accuracy of language assessment instruments and what our expectations should be regarding measurement and performance variability. In addition, previous research has demonstrated substantial instability in the classification of language impairment (LI) across the preschool and school years based on arbitrarily decided cut-points on standardized assessments. This study investigated the agreement between different age-based versions of a language assessment Address correspondence to: Patricia Eadie, Department of Audiology & Speech Pathology, University of Melbourne, 550 Swanston Street, Carlton, VIC 3053, Australia; e-mail: peadie@unimelb.edu.au International Journal of Language & Communication Disorders ISSN 1368-2822 print/ISSN 1460-6984 online C  2013 Royal College of Speech and Language Therapists DOI: 10.1111/1460-6984.12065