The interest rate for subsidized stafford loans is 3.4% (slightly above the prime rate). The rate for unsubsidized loans is twice that, at 6.8%. Subsidized loans are only available to undergraduates. Thus, graduate students in need of federal assistance must pay the much higher rate of 6.8% for unsubsidized loans.
In most cases, graduate students are unable to pay the unsubsidized interest each year and instead roll the interest payments into the loan principle. This can quickly become a crushing burden since graduate school often last six years (or more) for a PhD.
In a time when interest rates are record lows, graduate students should not have to pay such high rates.