Seems like you read this tutorial here. Have you tried this. Assuming your IP address is 123.456.78.90
:
iptables -s 123.456.78.90/32 -A INPUT -m statistic --mode random --probability 0.1 -j DROP
Or perhaps:
iptables -s 123.456.78.90/32 -p tcp -m tcp -A INPUT -m statistic --mode random --probability 0.1 -j DROP