If you want to identify records with blank values, you should use an OR statement because blank values can be represented in the Workbooks database in two forms:  

• NULL, which represents the absence of a value;

or

• An empty/blank field, which is a field-formatted value. 

To identify a field that is either blank or null, you should format your formula using an OR statement as shown below:

• column_abc IS NULL OR column_abc =''

NOTE:  If you use the predefined "is blank" or "is not blank" in reporting criteria or within Workbooks filters, Workbooks checks both options for you, thus 'under the covers':

• Choosing is blank is doing this:

  • column_abc IS NULL OR column_abc=''

• And choosing is not blank is doing this:

  • column_abc IS NOT NULL AND column_abc !=''
    
    

So, for example, if you want a report that shows you organisations that have a revenue in excess of £2,000,000 AND the number of employees field is empty, you would enter the formula as shown below:

IF( organisation_annual_revenue >2000000
AND (organisation_num_employees IS NULL
OR organisation_num_employees =''),
'Complies', 'Does not comply')

or you can use a built in Workbooks function that checks both blank and null for you, IS_BLANK:

IF( organisation_annual_revenue >2000000 
AND IS_BLANK(organisation_num_employees),
'Complies', 'Does not comply')

NOTE:  When looking for a blank value you should not enter a space between the two apostrophes, so it's '' and not ' '.