“…This effect is reflected in the positive estimated regression coefficients for WTP. Thus, for example, although for Amondo et al (2013) the income variable was not significant, Moffat et al (2011) and Roldán (2016) obtained positive coefficients with statistical significance at the 5% level, and Jalilov (2018), Cooper et al (2018), Wang et al (2015), Meibodi et al (2011), Diamini (2015, Kwak et al (2013)'s results were significant even at the 1% level. However, although many primary studies find this positive correlation between income level and WTP, results may differ when integrated into a meta-analysis because the ceteris paribus of each base study is not homogeneous, which implies that new variables emerge.…”