Both me and Philippa insist that it's easier to read an analogue clock-face (i.e. one with hands) than a digital clock-face. So I wondered: is there any research on the subject?
Of course there is! There's research about everything. But it doesn't seem to agree with us.
In Processing of visually presented clock times (Goolkasian, P and Park, D.C., 1980) the experimenters looked at the differences in speed for judging the time difference between two clocks, and found that "same/different reactions to digitally presented times were faster than to times presented on a clock face, and this format effect was found to be a result of differences in processing that occurred after encoding."
Minding the clock (Kathryn Bock, David E. Irwin, Douglas J. Davidson and W. J. M. Levelt, 2003) looked at explicitly linguistic effects (e.g. difference between Dutch and American English speakers). It also found that "responses to analog clocks were faster with relative expressions and responses to digital clocks were faster with absolute expressions," although overall it found again that digital clock-reading was faster than analogue in all cases. Note that the experimental method was explicitly linguistic - the speed measurements were measurements of how quickly the participants began to speak when they correctly named the time.
This is one of the most interesting (and most recent) results I found, partly because the experimental design included displaying the clocks for a short amount of time (as low as 0.1 seconds). 0.1 seconds is too quick for the eye to rove around the clock-face and fixate directly on the different parts of the display, and "the results from the 100 ms exposure conditions indicated that sufficient information for fairly accurate production can be extracted from the display without fixating the crucial information directly."
The effects of response format and other variables on comparisons of digital and dial displays (Miller R.J. and Penningroth S., 1997) "compared dial and digital clock displays to determine which could be read faster by 25 young adults" and found that "in general, digital displays led to faster responses than did dial displays. However, several combinations of the other variables, particularly those using the before-the-hour response format, effectively eliminated the superiority of digital displays. We suggest that in designing displays requiring such a response format, designers should not assume that a digital display is necessarily the best choice, especially if other factors encourage the selection of a dial display."
I haven't read the full paper (not available electronically; will have to visit my uni library) so I'm not sure if the experimental design was again based on participants reading the time out loud - and if so, I have an issue with that which I'll come to later. But this effect of before-the-hour responses is tantalising. For example: Philippa is a radio producer, and one of the things they need to do is glance at the clock to know how much time they've got before the programme ends at 6 o'clock precisely, so they can judge when to end interviews, when to bring in the next piece of music, etc. Philippa finds it much quicker to glance at an analogue clock in order to do this, and intuitively I can see why. You can literally see how much time is left (i.e. the size of the gap between the minute hand and the 12 o'clock mark), whereas with a digital clock you have to take in all the numbers and then do a quick arithmetic operation - not difficult, of course, but probably much slower, cognitively.
Judging a duration like this is very different from speaking the time. Reading out numbers is a one-to-one transformation which we do in so many contexts that it's very very easy; when reading out from a dial clock, we need to translate the hands' position into numbers before we can speak it. When using a dial clock to determine actions, however, we don't necessarily need to put the numerical step in the middle.
I'd like to run an experiment different to the ones I've found so far, one which tests the ability to comprehend clock-faces from a short glance - e.g. starting at 0.1 seconds and getting shorter. Rather than measuring the speed of vocalising, the measurement would be the minimum "glance time" for which the time could be correctly identified. My hunch is that the threshold will be a much shorter glance for analogue clocks.