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Rate of Change: The Simplest Momentum Measure

Rate of change, often written ROC, measures how much price has changed over a set number of periods, expressed as a percentage. It is the plainest way to quantify momentum.

The calculation is direct: today's close minus the close N periods ago, divided by the close N periods ago, times one hundred. A 12-period ROC on a daily chart tells you the percentage change over the last 12 days.

Positive values mean price is higher than it was N periods ago, and negative values mean lower. The zero line separates the two. The magnitude shows how strong the move has been.

Uses include trend confirmation, since sustained positive ROC supports an uptrend. Another use is comparing momentum across pairs, as the percentage figure is comparable between them, which helps rank which currencies are moving most. A third is spotting momentum loss, when price makes a new high but ROC does not, a form of divergence.

Because ROC is a lagging measure that depends on a single past price, it can jump when an old high leaves the window. That quirk is worth knowing, and smoothing with a moving average helps.

Momentum strategies in currencies have, in academic studies, shown modest returns over longer periods such as one to twelve months, though results vary by sample.

Practical step: compute the 20-day ROC for four major pairs, and rank them by strength.

Ranking pairs by ROC every weekend is a quick way to see which currencies are actually moving. I like it for picking which pair to focus on rather than for entries. A 20 day ROC on the main pairs takes a couple of minutes to check. If GBP shows strong positive readings against several currencies and JPY shows strong negative ones, GBPJPY or GBPUSD might be worth focusing on for trend setups that week. Pairs with ROC near zero are probably ranging, which suits a different approach. It's not precise, but it stops you spending the week on a pair that isn't going anywhere while something else trends cleanly.

The drop off effect is worth remembering. A sudden ROC jump can come from an old big candle leaving the window, not anything happening today. Say there was a large down day 12 days ago. When that day drops out of a 12 period window, ROC jumps up even if today's price barely moved. That can look like a momentum signal when it's just arithmetic. One way to reduce the effect is to smooth ROC with a short moving average, or to compare two lookback periods. If both show the same direction, the signal is more trustworthy. If only one moves, check what left the window.