People like formulas. They are expedient rules of thumb that guard against dangerous extremes. “Plan for one minute and a half per slide”, some say, “and never put more than 5 bullets point and more than 5 words per bullet point”. Under these rules lie hidden assumptions about people’s attention span, prior knowledge of the presented topic, text readability, number of clicks needed to go through the material on the slide, audience interactivity, and more! Presenters could be fooled into thinking that as long as these rules of thumb are followed, their presentation will be fine.
Rule of thumb #1: “Plan for one and a half minute per slide” is about as silly as telling a writer “Plan for chapters with 20 pages”. What is the purpose of this rule? It prevents presenters from putting so much information on one slide that to cover it would take more than 90 seconds. It also prevents boredom: people don’t generally like to stare at the same information for a long time. They get bored because they can read faster than the presenter can speak. Spending three minutes explaining each bullet point is as effective as administering a sleeping pill. What is important here is visual interest, not screen-time. A 30 second slide that gathers interest is fine. A two minute slide that exploits a particularly fruitful visual is fine so long as interest is maintained (let the audience be the judge of that through their questions). A slide that dynamically reveals and removes information through the use of layers can last a very long time, and it’s perfectly fine.
Rule of thumb #2: “Use not more than 5 lines and 5 words per line” (some say six lines, some say four; some say six words per line…). This is silly too, particularly in scientific presentations where long compound nouns abound. So what is the purpose of this rule? 1) to decrease the amount of text on a slide, so that the slide remains readable; 2) to prevent long lists that remove the need to try and select what is important and leave out what is less important; 3) to force the presenter to be concise as opposed to verbose; and 4) to allow the slide to be presented in less than 90 seconds; and thus maintain visual interest by not keeping the same slide on the screen too long – a point already covered above. What is important here is, again, the “Less-is-more” principle: the need to be selective to be legible, AND to be intelligible (clear), and finally, the need to keep visual interest with something other than words.
There is however one rule of thumb I like… but then again, because it works for me does not mean it works for you. This rule of thumb is based on your arm length and your palm size. It determines whether the text on your slide will be readable once projected on the large screen in front of your audience. Readability of text is not to be decided on the grounds that you can read everything on your PowerPoint slide, while sitting one foot away from your computer screen. High screen resolution and brightness will even allow font size 7 to be readable! So if your arm is long enough and your hand is not super tiny, and most of all, if you do not look like a chimp, this rule of thumb might work for you.
Start your slideshow. Stretch out your arm and turn your hand horizontally, fingers pointing to your left if you are right handed, and vice versa. Move away from your computer screen until your stretched hand hides the screen. Remove your hand (but do keep it at the end of your forearm) away from the screen. If you can read everything on the screen from that distance, chances are your audience will be able to read it too, once that slide is projected. Oh, by the way, if you really want to be sure, turn down the brightness of your screen to 50% and move back one half meter more 
Photo Flickr – by Lintmachine
