According to Slavin Creswell (2008:300) stated that there are
several keys ideas or characteristic to involving experiment research
that individuals. That key ideas as follow:
a) Random assignment
Random assignment is the process of assigning individuals
at random to group or to different group in an experiment. 
Random assignment helps increase in the personal characteristics.
So they have to learn and confident on his skill. Random
assignment helps increase the likelihood that any personal
characteristic that could bias a study’s outcome are evenly
distributed among groups of participants. The process of evenly
distributing potential bias is referred to as equating the groups.
It is important to note that random assignment and random
selection do not refer to the same process. Rather, random selection
refers to the process of selecting individuals so each individual has
an equal chance of being selected.   
b) Manipulation of treatment
True experimental research differ all other method of
research in that experimental research requires the manipulation of
at least one independent variable. This basically means that
researchers decide which variable will serve as the independent
variable to be manipulated and which group of participants will
receive this treatment”.