Measures of treatment effect

We anticipate that many of the outcomes will be continuous (e.g. pain, anxiety) and that varying measurement instruments will be used to measure the same underlying construct across the studies. For this reason, we will quantify the effects of aromatherapy using the standardised mean difference (SMD) (implementing the Hedges’ adjusted g version). In trials where a continuous measure has been dichotomised (e.g. a continuous pain scale is dichotomised into improvement or no improvement) and analysed as binary outcomes, we will re-express reported, or calculated, odds ratios as SMDs [60]. For dichotomous outcomes, we will quantify the effects of aromatherapy using risk ratios (RR). Given the wide range of conditions and outcomes in this review, it is not possible to specify specific thresholds for interpreting the size of the effect for each outcome. Given this, we plan to use Cohen’s guiding rules for SMDs where 0.2 represents a small effect, 0.5 a moderate effect and 0.8 a large effect [61]. Where a valid and reliable minimal important difference (MID) is available for a familiar measure of relevance to the population groups in the meta-analysis, we will re-express the SMD in units of the measure and interpret the effect in relation to the MID if feasible to do so [61]. For dichotomous outcomes, we will seek advice from the NTWC on interpreting the size of the effect (seeking agreement on a threshold for a small but important difference).