INT J LANG COMMUN DISORD, JULY –AUGUST 2012, VOL. 47, NO. 4, 427–436 Research Report Assessing the treatment effects in apraxia of speech: introduction and evaluation of the Modiﬁed Diadochokinesis Test Joost Hurkmans†, Roel Jonkers‡, Anne M. Boonstra†, Roy E. Stewart§ and Heleen A. Reinders-Messelink† †Rehabilitation Center ‘Revalidatie Friesland’, Beetsterzwaag, the Netherlands ‡Department of Linguistics, University of Groningen, Groningen, the Netherlands §University Medical Center Groningen, Groningen, the Netherlands (Received July 2011; accepted February 2012) Abstract Background: The number of reliable and valid instruments to measure the effects of therapy in apraxia of speech (AoS) is limited. Aims: To evaluate the newly developed Modiﬁed Diadochokinesis Test (MDT), which is a task to assess the effects of rate and rhythm therapies for AoS in a multiple baseline across behaviours design. Methods: The consistency, accuracy and ﬂuency of speech of 24 adults with AoS and 12 unaffected speakers matched for age, gender and educational level were assessed using the MDT. The reliability and validity of the instrument were considered and outcomes compared with those obtained with existing tests. Results: The results revealed that MDT had a strong internal consistency. Scores were inﬂuenced by syllable structure complexity, while distinctive features of articulation had no measurable effect. The test–retest and intra- and inter- rater reliabilities were shown to be adequate, and the discriminant validity was good. For convergent validity different outcomes were found: apart from one correlation, the scores on tests assessing functional communication and AoS correlated signiﬁcantly with the MDT outcome measures. The spontaneous speech phonology measure of the Aachen Aphasia Test (AAT) correlated signiﬁcantly with the MDT outcome measures, but no correlations were found for the repetition subtest and the spontaneous speech articulation/prosody measure of the AAT. Conclusions & Implications: The study shows that the MDT has adequate psychometric properties, implying that it can be used to measure changes in speech motor control during treatment for apraxia of speech. The results demonstrate the validity and utility of the instrument as a supplement to speech tasks in assessing speech improvement aimed at the level of planning and programming of speech. Keywords: apraxia of speech, diadochokinesis, speech therapy, improvement. What this paper adds What is already known on the subject Oral diadochokinesis is considered as a sensitive measure for the assessment of motor performance of speech. This paper describes the development of a modiﬁed diadochokinesis test, an instrument to evaluate rate and rhythm therapies. What this paper adds This study showed adequate psychometric properties of the modiﬁed diadochokinesis test with strong test-retest, intra- and inter-rater reliability, discriminant- and convergent validity. The instrument replenishes comprehensive speech tasks in assessing speech improvement. Introduction The present study describes the development and evaluation of a new instrument to evaluate treatment in apraxia of speech (AoS): the Modiﬁed Diadochoki- nesis test (MDT). Address correspondence to: Joost Hurkmans, Rehabilitation Center ‘Revalidatie Friesland’, NL-9244 CL Beetsterzwaag, the Netherlands; e-mail: j.j.s.hurkmans@revalidatie-friesland.nl Although the debate about the underlying deﬁcit in AoS is ongoing, there is agreement on at least some salient symptoms of this disorder. Ziegler (2008) characterized AoS as dysﬂuent, with groping and effortful speech, phonetic distortions, phonemic International Journal of Language & Communication Disorders ISSN 1368-2822 print/ISSN 1460-6984 online c  2012 Royal College of Speech and Language Therapists DOI: 10.1111/j.1460-6984.2012.00155.x