Supplementary Box 1 The ability of neurologists, radiologists and emergency

physicians to detect infarction or hemorrhage on cranial CT.

For full details of the study, see Schriger D et al. (1998) Cranial computed tomography interpretation in acute stroke. JAMA 279: 1293–1296.

A study tested 29 neurologists, 36 radiologists and 38 emergency physicians on their ability to read cranial CT scans. They were asked whether the patient shown in each CT would be suitable for thrombolysis, thereby testing their detection of hemorrhage or acute infarction. The scans showing hemorrhage came in two varieties—‘easy’ or ‘difficult’—and the scans showing infarct came in three varieties—‘easy’, ‘intermediate’ or ‘difficult’. The subjects were also presented with ‘normal’ and ‘imposter’ scans (calcification instead of hemorrhage and old infarction instead of acute infarction). Each scan was read a number of times by each type of clinician, giving a percent correct score for each type of scan. For the purposes of deriving a value of d-prime (d’) from these data, the numbers of correct readings at the different difficulty levels were summated. Hit rates were calculated as the number of scans correctly identified divided by the total number of readings. False-alarm rates were calculated by adding the number of normal scans correctly identified to the number of relevant imposter scans correctly identified (‘calcification’ for hemorrhage and ‘old infarction’ for acute infarction), and dividing by the total number of readings.

The emergency physicians correctly identified the ‘normal’ scan on 90 of 109 readings (83%) and the ‘calcification’ scan on 30 of 39 readings (77%). Their ‘correct rejection’ rate is therefore 120/148 = 0.81, and ‘false alarm rate’ is 1.0 – 0.81 = 0.19. They also correctly identified the ‘hemorrhage’ scan on 163 of 222 readings (73%), giving a hit rate of 0.73. We can calculate d’ as follows:

d’ = z(H) – z(F)

d’ = z(0.73) – z(0.19)

d’ = 0.61 – (–0.88) = 1.49

The other values for the emergency physicians, along the values for the other specialties, are shown in the table below.

Ability to detect
hemorrhage / Emergency
physicians / General
radiologists / Neurologists / Ability to detect
acute infarct / Emergency
physicians / General
radiologists / Neurologists
CT with noise / CT with noise
a) Normal / 90/109 =
0.83 / 87/98 =
0.89 / 73/81 =
0.90 / a) Normal / 90/109 =
0.83 / 87/98 =
0.89 / 73/81 =
0.90
b Calcification / 30/39 =
0.77 / 35/49 =
0.71 / 25/32 =
0.78 / b) Old infarct / 18/42 =
0.43 / 23/33 =
0.70 / 19/32 =
0.59
Correct rejection (CR) rate / 120/148 =
0.81 / 122/147 =
0.83 / 98/113 =
0.87 / CR rate / 108/151 =
0.72 / 110/131 =
0.84 / 92/113 =
0.81
False-alarm (F) rate / 1.0 – 0.81=
0.19 / 1.0 – 0.83 =
0.17 / 1.0 – 0.87 =
0.13 / F rate / 1.0 – 0.72=
0.28 / 1.0 – 0.84 =
0.16 / 1.0 – 0.81 =
0.19
CT with signal / CT with signal
a) Hemorrhage
(easy) / 92/98 =
0.94 / 78/78 =
1.00 / 69/69 =
1.00 / a) Acute infarct
(easy) / 37/45 =
0.82 / 27/29 =
0.93 / 34/34 =
1.00
b) Hemorrhage
(difficult) / 71/124 =
0.57 / 102/128 =
0.80 / 77/99 =
0.78 / b) Acute infarct
(medium) / 45/97 =
0.46 / 81/95 =
0.85 / 54/70 =
0.77
c) Acute infarct
(difficult) / 2/15 =
0.13 / 15/30 =
0.50 / 8/18 =
0.44
Hit (H) rate / 163/222 =
0.73 / 186/206 =
0.87 / 146/168 =
0.87 / H rate / 84/157 =
0.54 / 123/154 =
0.80 / 96/122 =
0.79
D-prime
z(H) – z(F) / 0.61 – (–0.88)
= 1.49 / 0.95 – (–1.13)
= 2.08 / 1.13 – (–1.13)
= 2.26 / D-prime
z(H) – z(F) / 0.10 – (–0.58)
= 0.68 / 0.84 – (–0.99)
= 1.83 / 0.80 – (–0.88)
= 1.68