MDX query with IIF

Hi,
my query is working with 1 parameter with values "yes" and "no". If user select "yes" I want only records with this value, if user select "no" I want to have both of them.
Has anyone idea how to do it?

My approach :

WITH MEMBER [BUSINESS TIME].[Avg No] AS iif (${paramBustime}='yes'),
[BUSINESS TIME].[yes], [BUSINESS TIME].[All BUSINESS TIMEs]
)

select {[Measures].[Occ perc]} ON COLUMNS,
{[WEEKDAY].[All WEEKDAYs].Children} ON ROWS
from [CUBE_AVG]
where crossjoin(Crossjoin({[ACTUAL DATE].[${paramDateFrom}]:[ACTUAL DATE].[${paramDateTo}]},
{[BUSINESS TIME].[Avg No]}), {[SECTION ID].[${paramSection}]})

... with "WITH MEMBER [BUSINESS TIME].[Avg No] AS iif (${paramBustime}='yes')" query is not working, with "AS iif ('yes'='yes')" or "AS iif ('no'='yes')"it works and deliver different results but I cannot get parameter in this query.

Thanks a lot for your help!
Best regards,
timfu83