2.5. Statistical analysis
Statistical analyses were performed with SPSS for Windows (version 19, IBM, Armonk, New York). Kolmogorov–Smirnov Test was applied first to test the normal distribution hypothesis for the data sets. Linear Mixed Model Analysis (Dependent variable: perceived effort ratings based on Borg CR10 Scale; Fixed factors: all other factors; Random factor: subject; Repeated covariance type: compound symmetry) was applied after that to test the fixed effects of the jar lid design characteristics (main effects and interactions). Associated pairwise comparisons (e.g. comparing estimated marginal means) were made if any significant effects were found; Tukey's test was used. The relationship between perceived effort and discomfort ratings was examined by Spearman Correlation Coefficient.