Variance Calculation

[#1]

Hi

I am trying to caluate the variance between the quarters in a year, I have used the expression below

=(avg( {$< Quarter = { 'Q1' } >}[Total Average]) - avg( {$< Quarter = { 'Q2' } >}[Total Average])) / avg( {$< Quarter = { 'Q2' } >}[Total Average])

but i need to include this conditoin ({$<[Directorate]={'Surgery'}>})

please help