University of Minnesota

How to avoid counting the same plant twice

(Photo: Wendy Caldwell)

If you are doing systematic sampling (i.e. counting every nth milkweed plant that you observe), plan your approach before you begin sampling to avoid sampling the same areas more than once. You do not have to start from the same location or follow the same paths each week, but make sure that you do not go through an area more than once. Think of every week or monitoring event as “starting fresh”. Don’t think about plants that you monitored the week before, and just start with the same systematic approach you’ve been doing all season. It is important not to bias your samples by going back to the same plants each week, especially if you remembered one that had monarchs on it.

If you use the random transect approach, you randomly choose a starting point and direction for your first transect across the site. As you continue using this method to generate new transects across the site, it is possible that you will intersect other transects that you have already monitored, and potentially sample the same milkweed(s) more than once. This is okay; just continue along the transect monitoring each milkweed along the way. The number of plants monitored twice will be few, and the random generation of transects through the site should still be representative of what is actually there. You can avoid overlapping transects if your transects are parallel to each other; once you hit the edge of your site with one transect, move a predetermined number of steps away from it, and start back in the opposite direction, parallel to the first transect.

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