Every keyword arrives with numbers attached, and the numbers are more uncertain than they look. Monthly search volume, often written as MSV, is a model rather than a count. Difficulty is a vendor's score with no agreed definition. Cost per click is real money from a different market. Used as an ordering, all three are useful; used as thresholds, they produce confident decisions built on estimates nobody can check.
Monthly search volume is a modelled average
MSV is presented as searches per month, and it is usually an average over the past year, modelled from sampled data and rounded into bands. Two consequences follow. Seasonal queries look flat when they are anything but, so a term that gets almost all its searches in one quarter reads as a modest steady number. And two tools will disagree, sometimes by a lot, without either being wrong in the way people expect.
Difficulty is a score, not a measurement
Every difficulty number is a vendor's model of how hard a query looks, usually built from the strength of the sites currently ranking. It knows nothing about your site's standing, nothing about how well the existing pages actually answer the query, and nothing about whether the intent is one you can serve better. Use it to sort a long list quickly; do not use it as a gate, and never compare a score from one tool with a score from another.
Cost per click is the most honest number of the three
CPC comes from an auction where advertisers spend real money, so it carries information the other two do not: what a click on this query is worth to somebody who has measured it. It is not your value per click, and it reflects a paid market rather than an organic one, but as a signal of commercial intent it is far better than guessing from the words.
What to do with all three
Use them to order candidates, then decide with your own arithmetic: what a click is worth to you, what share of the traffic you could take, and what conversion rate you actually see. That is the sum the free worksheet on this site does, and its inputs are deliberately yours, because the estimate you can check beats the estimate you cannot.
Questions people ask about keyword statistics
What does MSV mean in SEO?
Monthly search volume: a modelled estimate of how often a query is searched in a month, usually averaged over a year and reported in bands rather than exact counts.
Why do keyword tools disagree on volume?
Each builds its estimate from its own sampled data and its own model. Pick one source for a whole list, because the ordering within the list is what you act on.
Is keyword difficulty reliable?
As a rough sort, yes. As a threshold, no: it is a vendor's model that knows nothing about your site or how well the ranking pages actually answer the query.
Which number should decide whether to write a page?
None of them alone. Combine them with your own conversion rate and value per conversion, which is the arithmetic the free worksheet here does.