Comparing Means of Data for multiple Years

My research project surveyed 120 or so aquatic ponds for the presence of the Natterjack Toad. Some of these were natural ponds and the others were constructed.
I want to compare my results to previous surveys results. I have egg string counts for each pond and I have a list of these and the ponds containing egg string counts. However, some ponds were surveyed in 2016-2018 for example. These ponds were excluded from the 2021 survey as those landowners were no longer participating in the pond construction scheme that I am monitoring as part of my project.
In my table I have the list of ponds and the total number of egg strings for each pond for every survey year (2006-2007, 2011-2012, 2016-2018 and 2021). However due to the absence of some ponds from the previous surveys in the current survey there are no results. For these ponds do I record the egg string count as 0 under 2021 even if that pond mightn't have been assessed. Or do I just leave the cell as a missing value?

I hope you get what I mean, I can try and explain further if required. Please see the photograph below:
A # denotes the ponds that were not surveyed in the study year as described above. For Example M15 was surveyed in 2011 and 2012 but not in 2016, 2017, 2018 or 2021. Should the # be replaced with a zero even if these ponds were not surveyed or should the cells be left empty as they were not surveyed for an egg string count?

Hope you can help!


Active Member
most software will expect this to be left missing. you might impute to 0 in some methodology, but it is not preferred.