"seen on average" is correct and usable in written English.
You can use it when you want to refer to something that can be observed as a pattern or trend, usually as a result of a review or study. For example: The study revealed that students were seen on average to experience a decrease in stress when taking part in physical activities twice a week.
During the interview, we asked residents how many cockroaches they had seen, on average, each day over the previous 3 months and assigned responses to one of six categories ranging from 0 to ≥ 20 per day.
The planet's disk was seen, on average, at an angle of 7.7″.
During that period we have seen on average around 200,000 detections per day.
He said: "The survey data are consistent with the region's GDP growing at a quarterly rate of 0.3% in the third quarter, or 1.2% annualised, which is similar to that seen on average over the first half of the year".
Michael A. Petralia, chief of public and governmental affairs for the Port Authority of New York and New Jersey, which paid more than $120,000 to create and install the panels, estimated that they are seen, on average, by more than 20,000 visitors a day.
As I was later to discover, the concentrated rain as much as 0.54 of an inch in a single five-minute period, an intensity seen on average just once every two hundred years propelled a debris flow down the slopes of the denuded mountains, the first indication of which was that fiery glow in the sky.
The two Republicans saw on average more than a 2 percentage-point increase.
Respondents reported seeing on average seven patients per week whom they believed to have HIV.
The service, which is centered around streaming live videos of people playing video games, sees on average one million broadcasters and 45 million viewers per month.
"Our kids see thousands of food ads for products loaded with sugar, fat and salt and see on average just one ad a week for healthy products like water, fruit and vegetables," she points out.
It occurs rarely in primary care: a full time general practitioner (GP) would see on average only one case every 5 years (NICE, 2001).
