Background A growing number of studies report cognitive impairment after chemotherapy; indeed the phenomenon of chemo-fog is now almost universally accepted. We are concerned however that there is little if any consistency in the way in which patients are classified as showing cognitive impairment or not. We aim to demonstrate that different methods of analysis produce markedly different results, making the true extent of impairment unclear. Methods We analysed data from 92 breast cancer patients 4 weeks post-chemotherapy and from 42 healthy controls using 7 different methods, each taken from a different research paper in the area of cognitive impairment post-chemotherapy. Findings The extent of impairment was dependent on the method of analysis. Impairment ranged from 12 to 68.5% in the chemotherapy group and from 4.8 to 64.3% in the healthy control group. Interpretation This brief report highlights the contrasting degrees of cognitive impairment calculated by using legitimate statistical methods and demonstrates the need for a collaborative effort to standardise our methods that we might better understand the phenomenon of chemo-fog.