As this was new for me I Googled it. Wikipedia says this, which has left me more confused:

"About 4% of people have one of the types of bipolar disorder at some point in their life.[101] Lifetime prevalence of bipolar disorder type I, which includes at least one manic episode during a lifetime, has generally been estimated at 2%.[102] However, a reanalysis of data from the National Epidemiological Catchment Area survey in the United States suggested that 0.8% of the population experience a manic episode at least once (the diagnostic threshold for bipolar I) and a further 0.5% have a hypomanic episode (the diagnostic threshold for bipolar II or cyclothymia)."

Doesn't this mean that bipolar I is as common or more common than bipolar II? (I thought bipolar I was less common, but I thought the prevalence for all of them was higher. I'm assuming the reanalysis is a better analysis) It would be interesting to see data on how often a bipolar dx is changed because of increased severity of symptoms.