How is the percentage calculated?
Against the visits that already match the selected criteria, not against the workspace total. A figure of 40% for brand B means that in 40% of the visits where your brand was analysed, B was too.
How many visits before the data is reliable?
There is no magic number, but below a few dozen visits per brand the pairs swing a lot. The sign that you have enough is when the ordering of brands stops changing week to week.
Is co-occurrence the same as share of shelf?
No. Share of shelf measures space held; co-occurrence measures joint presence. They complement each other: the first says how much, the second says next to whom.
Do I need to analyse competing brands in visits?
Yes, that is the condition for having any data. If the checklist only covers your own brands there are no pairs to record. It is enough to include the main competitors as catalogue brands.
Does this work in the online channel?
It does, and it is usually more revealing there, because a category page shows dozens of brands at once, which rarely happens on a physical shelf.
Is the data comparable across periods?
It is, as long as the checklist and the brand set stay stable. Adding brands to the catalogue midway changes the comparison base, and it is worth recording when that happened.