Simulations of structure in our residuals of PSR B1919+21. The top panel shows an example of our TOAs, with a simulated ν^-4 ISM-like delay, which would be 0.84 ms at 48 MHz, added to them. The fit to the data is shown by the black line, and the null hypothesis (no ISM delay) is plotted in grey. Similarly, the right hand panel shows an example of a simulated aberration/retardation-like delay with a ν^0.6 dependence which is 0.28 ms at 180 MHz. Because the errors on the TOAs are much smaller at high frequencies, we are much more sensitive to delays at high frequencies, despite the fitted jump. Note that the sensitivity of both the LOFAR LBA and HBA observations is now vastly improved and so we should be able to better constrain (or even detect) some of these effects in the near future.