Casing treatments are used to extend the stability margin of fans or compressors. These typically have an axisymmetric arrangement, but for a fan exposed to non-axisymmetric distortion, a casing treatment applied to a fraction of the annulus is of interest to target a particular distortion feature. In this paper, a partial casing treatment has been designed with axial-skewed slots, and an experimental investigation has been carried out to determine the effects of the treatment extent and position. Three inlet conditions were tested for each extent of casing treatment: clean, radial tip-low distortion, and half-annulus tip-low distortion. For axisymmetric inlet conditions, the stall margin and efficiency penalty increased approximately linearly with casing treatment extent. However, for half-annulus tip-low distortion, there was a strong dependence between the position of the casing treatment relative to the distorted sector and the stall margin improvement. If the casing treatment is positioned where the rotor enters the distorted sector, unsteady disturbances in the rotor tip flow are minimized. However, if the rotor meets the casing treatment later in the distorted sector, the disturbances are able to grow, leading to earlier stall inception. Overall, this shows that partial casing treatments can target a distorted flow to give a disproportionately positive impact compared to a full annulus casing treatment. For example, it was found that a 72 deg casing treatment extent could be positioned to achieve over 50% of the full annulus casing treatment stall margin improvement with only 20% of the efficiency penalty.