OBJECTIVES:This study sought to show the degree to which experience and training affect the precision and validity of global longitudinal strain (GLS) measurement and to evaluate the variability of strain measurement after feedback. BACKGROUND:The application of GLS for the detection of subclinical dysfunction has been recommended in an expert consensus document and is being used with increasing frequency. The role of experience in the precision and validity of GLS measurement is unknown, as is the efficacy of training. METHODS:Fifty-eight readers, divided into 4 groups on the basis of their experience with GLS, calculated GLS from speckle strain analysis of 9 cases with various degrees of image quality. Intraclass correlation coefficients (ICCs), mean difference, SD, and coefficient of variation (CV) were compared against the measurements of a reference group that had experience with >1,000 cases of strain measurement. Individualized feedback was distributed, and repeat measurements were performed by 40 readers. Comparisons with the baseline variation provided information about whether feedback was effective. RESULTS:The ICC for GLS was significantly greater than that for ejection fraction regardless of image quality. Experience with strain measurement affected the concordance in strain values among the readers; the group with the highest level of experience showed significantly better ICC than those with no experience, although the ICC of the inexperienced readers was still very good (0.996 vs. 0.975; p = 0.0002). As experience increased, the mean difference, SD, and CV became significantly smaller. The CV of segmental strain analysis showed significant improvement after training, regardless of experience. CONCLUSIONS:The favorable interobserver agreement of GLS makes it more attractive than ejection fraction for follow-up of left ventricular function by multiple observers. Although experience is important, the precision of GLS was high for all groups. Training appears to be of most value for the assessment of segmental strain.