The Detention-Rate Paradox: What the Enforcement Effectiveness Index Reveals About Port State Control

Port State Control has one headline number, and almost everyone reads it the same way: a low detention rate means a clean, well-run port, and a high one means trouble. The Enforcement Effectiveness Index (EEI), applied to 129,493 inspection records across 667 ports in the Tokyo, Paris, and Caribbean MoUs over four years, shows that this intuition is not merely incomplete. It is frequently backwards.

The EEI asks a question the detention rate does not. For every vessel a port inspects, it looks at whether that ship leaves with fewer deficiencies than it carried at its previous inspection, or more. The index for a port is simply the share of vessels that improved minus the share that worsened, on a scale from +100 to −100. It is a plain, reproducible measure of the one thing that should matter most: at this port, do the ships that pass through tend to get better, or worse?

The answer reorders the field. The detention rate explains less than a quarter of the variation in the EEI, which means roughly three-quarters of what distinguishes an effective port from an ineffective one is invisible to the metric the entire system relies on. Nearly two-thirds of ports (64.9%) recorded no detentions at all across the full period. Those zero-detention ports posted a negative average EEI: across the vessels they inspect, deterioration outpaces improvement. Ports that do detain averaged solidly positive. The gap between the two groups is large by any statistical standard. A port that never detains, in other words, is on average not a clean port at all; it is often one applying no corrective pressure whatsoever.

The most consequential finding sits inside that pattern. A group of 169 ports, accounting for tens of thousands of inspections, and including high-volume hubs such as Callao, Port Hedland, and Ho Chi Minh City, looks immaculate on every conventional measure, with no detentions on record, while ranking in the worst quartile for vessel deterioration. They are invisible to commercial vetting platforms and to MoU secretariats alike, precisely because they score well on the only thing those systems currently watch.

Cluster analysis sorts the world's ports into three types. Enforcement-Active ports detain meaningfully and improve the vessels they inspect, the benchmark, and overwhelmingly Paris MoU facilities. Enforcement-Passive ports, the largest group, inspect in volume but apply no teeth: vessels drift into worse condition, and this is where every one of the hidden-risk ports lives. The third type is the most uncomfortable: High-Sanction/Deteriorating ports that detain at the highest rate of any group yet still send vessels away worse, and they operate under the very same Paris MoU regime and targeting algorithm as the best performers, sitting roughly 46 EEI points below them. Detention activity and enforcement effectiveness, it turns out, can point in opposite directions at the same time.

A word on what the index does not claim, because the discipline is the point: the EEI is a descriptive association, not proof of cause. Fleet composition, drydocking schedules, and survey cycles all shape a vessel's deficiency trajectory, and the paper is explicit about that limit rather than papering over it. What the EEI offers MoU secretariats and the IMO is an outcome-based indicator they can compute from data they already hold; what it offers charterers, vetting, and P&I is a port-risk signal the detention rate cannot give; and what it offers classification societies is a map of where their classed tonnage is quietly degrading between calls.

Plimsoll Analytics built the EEI to evaluate enforcement by its outcomes rather than its procedures to answer, port by port, whether ships leave better or worse than they arrived.