Influencer marketers face a bewildering array of choices when planning a campaign. How many influencers should we use? How much should we pay them? Should we pay them anything? Should we be very prescriptive or give free rein to their creative intuition?
A recurring challenge is whether to use just a few influencers with a large following, or lots of so-called ‘micro influencers’, who have a smaller but very engaged fan base. Or a mix of both. Received wisdom is that smaller influencers have a more loyal, active following, who anticipate and engage with all the content they publish. They are also cheaper, sometimes free. Leveraging these influencers should lead to greater overall campaign engagement and is more likely to lead to conversions and ROI.
But is this received wisdom true? If so, to what extent? Does it apply in the Beauty as well as the Travel vertical? Does it apply on branded content as well as non-branded? Are the greater search and management overheads incurred working with many less well organised, less professional influencers worth it for a negligible gain in campaign engagement?
In this blog post we delve into the data to uncover the relationship between engagement and account size. We explore the trade-off between using micro and macro influencers and give benchmarks for what engagement performance is typical given influencer account size and vertical. We focus on Instagram and analyse a representative 300,000 branded and non-branded posts published over the past 6 months by 3,000 influencers with between 10k and 3m followers.
What is engagement?
Firstly, what do we mean by engagement? On Instagram you can engage with a post in two ways: by ‘liking’ or by writing a comment. The goal of most influencer campaigns is to generate as much engagement as possible, as this is assumed to be correlated in the longer or shorter term with growing awareness or, more directly, with sales.
Posts by influencers with more followers are seen by more users and therefore typically generate a greater number of engagements:
More followers means more engagement. 
But more followers usually also means more cost. A (very) rough rule-of-thumb is a penny a follower a post. So a post from an influencer with 500k followers would cost £5k.
 It should also be noted that causality can run the other way, too: a very engaging post will draw in new followers. This chart takes follower count at the time of post publication and the latest engagement count.
Finding bang for buck
The metric most often used to measure campaign success is engagement rate. A higher engagement rate means more engagements given the number of followers, and also therefore given the cost.
We calculate an engagement rate by adding up likes and comments and dividing by the number of followers:
This is the number of engagements per follower per post, i.e. value for money.
To maximise the total number of engagements a campaign generates for a given budget, you need to maximise the engagement rate.
Engagement rate vs. account size
Many interacting factors affect the engagement rate a post achieves. There’s random variation, caused by peculiar qualities of the post, or Instagram’s algorithms, or world events that day. There are seasonal and cyclical trends, and fads. If an influencer does too many branded posts it’s possible there’s a dilution effect, whereby they lose credibility and their audience switches off.
A consistent contributing factor is the size of the account. A given percentage increase in the number of followers leads to a smaller percentage increase in engagements, and therefore a fall in the engagement rate.
If we fit a line to the chart above and look at the relationship between engagement rate and followers we get this:
Note there is A LOT of noise around this line. Many posts from accounts with a large following will outperform posts from micro influencers.
Brand-sponsored posts tend to perform less well than non-branded posts. Here we group influencers by their size and take an average of their branded engagement rates:
There is a big difference between very small influencer performance (10k - 25k) and very large (1M - 5M), but no significant difference between 100k and 1M. Note that there is most variation in performance in the 100k - 500k and 500k - 1M groups; some influencers in these groups perform very well, others constantly poorly.
Branded post performance by vertical
Here we give some example benchmarks from our database for branded post performance by vertical by account size:
These numbers demonstrate the trade-off between size and performance: larger accounts generate more engagements, but lower engagement rates, and so fewer engagements per £ of investment.
In practice account size is one of many factors planners must weigh up. It is not always the case that many micro influencers will provide a greater return on investment than a handful of juggernauts. When evaluating campaigns, benchmarks are useful to gauge relative performance and the more granular the better. Much is down to chance, but with more data and greater insight, influencer marketers can push up the return on their investments.
CampaignDeus is the leading independent provider of influencer marketing campaign data for Instagram and YouTube. Our platform identifies and classifies brand sponsored influencer campaign performance metrics, tracking hundreds of thousands of posts.
We use this data to provide Brands & Agencies with industry insights across verticals, benchmark campaigns against vertical & competitor averages, and equip clients with in-depth reporting and recommendations on how to make campaigns more effective. Get in touch for more details.
Duncan Stoddard is the co-Founder and Chief Data Officer of CampaignDeus. Previously a data scientist with AlphaSights and media agency Mindshare, he specializes in predictive modelling, forecasting, optimisation and data visualisation.