One way to verify that your calculator is working properly is to follow the example on Page 89. To get mean, you should use the orange shift key followed by [ 7 ] and the answer 3.5 should display.

To get Standard Deviation, press orange shift key followed by [ 8 ] and you should get the answer 0.85.

I have attached the quick reference screen print from Page 89 for your HP 10bII Manual.

Screen Print