Interventions are used in response to a particular diagnosis or problem faced by a patient population.  The goal of health research is to measure and improve the effectiveness of particular interventions or treatments on that population.  

When selecting your intervention (I) in your PICO, make sure to choose an appropriate intervention for the population (and diagnosis).  For instance, supported education might be a great intervention to address the problem of adults with schizophrenia not completing college, but it will not be appropriate to address cross infections in hospitals.

Research studies need to be controlled experiments, so each research article will examine the effect of one particular intervention on a given population.  When planning the intervention part of your PICO, choose only one intervention to examine.  

Comparison is useful in measuring the impact of your intervention because it provides context.  The C in PICO means an alternative to the intervention (I) that you are trying to measure.  

Your comparison may be an alternative intervention, but it may also be the absence of intervention.  For instance, when measuring the effect of increased hand washing as an intervention, it is good to compare it to no increase in hand washing.  Similarly, if you want to measure the effect of an infrared device such as Vein Viewer on the placement of PICCs in babies, your comparison could be with placement of PICCs without Vein Viewer.  This is how you will recognize the value of the intervention you are considering.

Just like interventions, make sure you only pick one comparison for each PICO.