Shell Programming and Scripting

I have written a script that will calculate CPU usage and Memory usage of a particular process on AIX Unix. I know the PID and I am using the command --

# for CPU Usage
MSG_CPU_USG=`ps uax | grep ${PID} | awk {'print $3'}`

#for Memory Usage
MSG_MEM_USG=`ps uax | grep ${PID} | awk {'print $4'}`

This unfortunately does show 0 always even with heavy load to the process.
The process spawns threads.
Is the command correct to use?
Can you please help me?

Thanks in advance,
Asutosh