```
julia> a=[true,false,true,true,missing]
5-element Vector{Union{Missing, Bool}}:
true
false
true
true
missing
julia> mean(skipmissing(a))
0.75
julia> length(a)
5
julia> length(skipmissing(a))
ERROR: MethodError: no method matching length(::Base.SkipMissing{Vector{Union{Missing, Bool}}})
```

I want to calculate the length without missing in a,and only count the true and false ,then return 4.What should I do ? Thanks for helping me.