* Definition of bullying: Studies use different definitions, encompassing everything from physical aggression to cyberbullying and social exclusion. A broader definition will naturally lead to a higher percentage.
* Methodology: How the data is collected (self-reports, teacher reports, peer reports, observations) significantly impacts the results. Self-reporting can underestimate bullying due to shame or fear of retaliation, while teacher reports might miss subtle forms of bullying.
* Geographic location: Bullying prevalence differs across countries and even within regions of a country due to cultural factors and school environments.
* Age group: Bullying rates can vary within the primary school years (typically ages 5-11).
While precise figures are elusive, many studies suggest that a substantial portion of primary school children experience bullying, often estimated to be in the range of 15% to 30% or even higher depending on the factors mentioned above. It's crucial to remember that these are estimates, and the actual experience of bullying is complex and multifaceted.