s ) ▁are ▁not ▁measured ▁reli ably ▁( i . e . ▁with ▁large ▁amounts ▁of ▁measurement ▁error ), ▁incorrect ▁conclus ions ▁can ▁be ▁drawn . ▁ ▁Rest riction ▁of ▁range ▁Rest riction ▁of ▁range , ▁such ▁as ▁floor ▁and ▁ce iling ▁effects ▁or ▁selection ▁effects , ▁reduce ▁the ▁power ▁of ▁the ▁experiment , ▁and ▁increase ▁the ▁chance ▁of ▁a ▁type ▁II ▁error . ▁This ▁is ▁because ▁correl ations ▁are ▁at ten u ated ▁( weak ened ) ▁by ▁reduced ▁vari ability ▁( see , ▁for ▁example , ▁the ▁equation ▁for ▁the ▁Pear son ▁product - m oment ▁correlation ▁coefficient ▁which ▁uses ▁score ▁variance ▁in ▁its ▁estimation ). ▁ ▁H eter ogene ity ▁of ▁the ▁units ▁under ▁study ▁Gre ater ▁heter ogene ity ▁of ▁individuals ▁particip ating ▁in ▁the ▁study ▁can ▁also ▁impact ▁interpret ations ▁of ▁results ▁by ▁increasing ▁the ▁variance ▁of ▁results ▁or ▁obsc uring ▁true ▁relationships ▁( see ▁also ▁sampling ▁error . ▁, ▁the ▁higher ▁the ▁standard ▁deviation ▁will ▁be . ▁This ▁obsc ures ▁possible ▁interactions ▁between ▁the ▁characteristics ▁of ▁the ▁units ▁and ▁the ▁cause - effect ▁relationship . ▁ ▁Th re ats ▁to ▁internal ▁valid ity ▁Any ▁effect ▁that ▁can ▁impact ▁the ▁internal ▁valid ity ▁of ▁a ▁research ▁study ▁may ▁bias ▁the ▁results ▁and ▁impact ▁the ▁valid ity ▁of ▁statistical ▁conclus ions ▁reached . ▁These ▁thre ats ▁to ▁internal ▁valid ity ▁include ▁un re li ability ▁of ▁treatment ▁implementation ▁( lack ▁of ▁standard ization ) ▁or ▁failing ▁to ▁control ▁for ▁extr aneous ▁variables . ▁ ▁See ▁also ▁ ▁In ternal