Custom Data Validation Lists

Hi All,

I do lot of data analysis related to Incidents worked by our team and generate reasons on why we missed or customer was not happy with helpdesk service. Basically may analysis will have two columns namely "Primary Reasons" and "Observations"
Primary Reasons generally fall into one of the following like:
1) Agent Mistake
2) Communication Issue
3) Outside GSD control
4) Resolution Time
5) No commnets provided

I need assistance in building data list so when Agent Mistake is selected under primary reason, observation column should only reflect list related to Agent Mistakes like Misinterpret the issue
Missed Call back
Improper follow Up
Incorrect KB followed
Delay in Responding to user
Probing Issue
Missed basic troubleshooting
Attachment Missing
Incorrect Routing

Similarily if Communication Issue is selected under Primary reasons, Observation should list one of the options to select like
Accent Issue
Active Listening
Soft Spoken
Difficult to understand
Zargons / Technical terms used

I am attaching file so that I could assistance. Data Validation sheet has above values selected and drop down list to be generated in Sample Work Sheet.