Half-life is the name of a concept commonly used in physics, that I include in my list of relevant mental models for consultants. Those mental models are ideas that you can use to interpret or explain things, improve your reasoning, and make better decisions.
A half-life is the time it takes for something to halve its quantity.
The original term, half-life period, comes from nuclear physics and describes how quickly unstable atoms decay or how long stable atoms survive. For example, if there are 100 radioactive atoms, and their half-life is one second, there will be only 50 left after one second. 25 after two seconds. And 12 after 3 seconds.
Different elements or substances have different half-life periods. For example, the biological half-life of water in a human being is about 9 to 10 days. The half-life of caffeine is around 6 hours - meaning it takes around 24 hours for a dose of caffeine to fully leave the body.
The number is an approximation - there's a random variation in the process. But the more homogeneous the atoms are, the better the half-life period will be as an estimation tool.
For us consultants, this concept becomes useful when we apply it to other disciplines, such as marketing, economics, or psychology.
Half-Life In Marketing
For marketers, the half-life of a campaign can mean the time taken to receive half the total responses. Of course, this number changes based on the kind of channel and media used.
Direct mail (a paper catalog, for example) may have a half-life of about three weeks, whereas a LinkedIn post might have a half-life of 90 minutes. This time can be used to calculate how frequently a message should be sent.
Half-life is also strongly linked to the concept of advertising adstock (or advertising carry-over), which is the prolonged effect of advertising on consumer behavior.
The idea is simple: Every new exposure to an ad reminds the consumer about a brand. The more recent is the exposure, the higher this awareness level will be. In the absence of new exposures, the awareness level gradually comes down - up to a point where your target audience forgets about you and your brand.
Some Interesting Results
One relevant and recent meta-analysis on carry-over effects is this one. Apart from confirming that the prolonged effects of advertising are getting shorter and shorter over time (which is expected due to the huge volume of ads we're exposed to), they measured and compared the half-life of traditional outbound initiatives - mass advertising, targeted advertising, and personal selling.
The half-life of personal selling was 6 times longer than the one from mass advertising, which highlights the importance of human interaction. But we consultants should take these results with a grain of salt. Unfortunately, this meta-analysis only looked at the promotion of products and didn't include social media or any inbound initiatives such as blogging and word-of-mouth.
Due to the volume and simplicity to track results, the half-life of email marketing has been extensively covered by digital agencies and SaaS companies. When we look at average numbers across different industries, half of your emails will get opened after 6 hours. And around 20-25% will get opened in the very first hour after sending.
An Argument For Specialization
Another interesting exercise is to think about the half-life of facts: how long does it take for half of the ideas in a field to be replaced or disproved?
This is the central question behind In "The Half-Life of Facts: Why Everything We Know Has an Expiration Date", by Samuel Arbesman. He gives the example of a dermatology test that had the same question two years running, with a different answer each time. The quantity of scientific data available doubles every 15 years, so much of it gets debunked or becomes obsolete.
But if information has a half-life, so do careers. Modern estimates the half-life of an engineering degree is between 2.5 and 5 years. Staying up to date would require 10 to 20 hours of study per week.
To be the best at what you do, you must not only cultivate unique expertise but also catch up on your field's latest developments and ideas. Your time and energy are limited. But you can improve your chances by specializing in a narrow niche.