I went for a walk today just using the vivofit (no hrm), I wanted to explore why I was getting the occasional weird step count.

I briskly walked 100 steps (counted them in my head) - vivofit read between 60-70 steps (did this numerous times and all readings were between these marks)

I ran 100 steps - readings between 98-100 (again numerous times)

I walked average speed - readings between 98-100

I walked briskly with my arms bent (similar to running) - readings between 98-100 - hmmmm

I walked briskly with my hands in my pocket - readings between 98-100 - hmmm I might be on to something

I walked briskly with my arm held stiff down by my side - readings between 98-100 - definitely on to something

I walked briskly swinging my arm freely - readings between 60-70 - yip looks like the arm swing interferes with the readings

I walked briskly with smaller controlled arm swings - readings between 98-100 - Jackpot, problem isolated.

It appears I have been losing around 3000 steps for every hour of brisk walking I have been doing - not nice.
If anyone has the time (and the inclination) could you also test out the brisk walk with arm swing to see if it affects your vivofit.
Thanks.