Vickers AD, Palmer SCF. The influence of canopy cover and other factors upon regeneration of Scots pine and associated ground flora within Glen Tanar NNR. Forestry (Lond). 2000;73(1):37-49.
Williams VSL. The "unbiased" anchor: bridging the gap between DIF and item bias. Appl Meas Educ. 1997;10(3):253-67. doi: 10.1207/s15324818ame1003_4