Supporting thinking on sample sizes for thematic analyses: a quantitative tool

Thematic analysis is frequently used to analyse qualitative data in psychology, healthcare, social research and beyond. An important stage in planning a study is determining how large a sample size may be required, however current guidelines for thematic analysis are varied, ranging from around 2 to over 400 and it is unclear how to choose a value from the space in between. Some guidance can also not be applied prospectively. This paper introduces a tool to help users think about what would be a useful sample size for their particular context when investigating patterns across participants. The calculation depends on (a) the expected population theme prevalence of the least prevalent theme, derived either from prior knowledge or based on the prevalence of the rarest themes considered worth uncovering, e.g. 1 in 10, 1 in 100; (b) the number of desired instances of the theme; and (c) the power of the study. An adequately powered study will have a high likelihood of finding sufficient themes of the desired prevalence. This calculation can then be used alongside other considerations. We illustrate how to use the method to calculate sample size before starting a study and achieved power given a sample size, providing tables of answers and code for use in the free software, R. Sample sizes are comparable to those found in the literature, for example to have 80% power to detect two instances of a theme with 10% prevalence, 29 participants are required. Increasing power, increasing the number of instances or decreasing prevalence increases the sample size needed. We do not propose this as a ritualistic requirement for study design, but rather as a pragmatic supporting tool to help plan studies using thematic analysis

Effectiveness and cost-effectiveness of humanistic counselling in schools for young people with emotional distress (ETHOS): study protocol for a randomised controlled trial

Background: One in ten children in Britain have been identified as experiencing a diagnosable mental health disorder. School-based humanistic counselling (SBHC) may help young people identify, address, and overcome psychological distress. Data from four pilot trials suggest that SBHC may be clinically effective. However, a fully powered randomised controlled trial (RCT) is needed to provide a robust test of its effectiveness, to assess its cost-effectiveness, and to determine the process of change. Methods/design: The Effectiveness and Cost-effectiveness Trial of Humanistic Counselling in Schools (ETHOS) is a two-arm, parallel-group RCT comparing the clinical and cost-effectiveness of SBHC with Pastoral Care as Usual (PCAU) in school settings. Eligibility criteria for young people include being between 13 and 16 years of age and experiencing moderate to severe levels of emotional distress. Participants are randomised to receive either SBHC or PCAU. SBHC is delivered in up to 10 weekly, individual sessions in their school with a qualified, experienced counsellor who has also received training using a clinical practice manual. Adherence to the SBHC model is assessed by a sub-team of auditors and in clinical supervision. PCAU consists of the schools’ pre-existing systems for supporting the emotional health and well-being of students. The primary outcomes are psychological distress measured using the Young Person’s Clinical Outcomes in Routine Evaluation (YP-CORE) and costs evaluated using the Client Service Receipt Inventory (CSRI). Secondary outcomes include psychological difficulties, levels of depression, anxiety and self-esteem, well-being, school engagement, educational outcomes and achievement of personal goals. Qualitative interviews with participants, parents and school staff will look to identify the mechanisms of change in SBHC. Researchers administering the measures are blind to allocation. The trial requires n = 306 participants (n = 153 in each group), with 90% power to detect a standardised mean difference (SMD) of 0.5. An intention-to-treat analysis will be undertaken. Discussion: This RCT is powered to detect clinically meaningful differences, and will make a major contribution to the evidence base for mental health provision for adolescents. It will have implications for all stakeholders, including policy-makers, statutory advisory bodies for child welfare, head teachers, children and young people practitioners, child welfare and parenting organisations, and young people. Trial registration: Controlled Trials International Standard Randomised Controlled Trial Number (ISRCTN) Registry, ID: ISRCTN10460622. Registered on 11 May 2016