QuanTAlib/lib/trends/mgdi/Mgdi.md

MGDI: McGinley Dynamic Indicator

"John McGinley saw moving averages failing in fast markets and said, 'It's not the market's fault, it's the math's fault.' MGDI is the apology."

MGDI (McGinley Dynamic Indicator) looks like a moving average, but it's actually a smoothing mechanism that adjusts itself relative to the speed of the market. It was designed to solve the problem of "lag" and "whipsaw" simultaneously by using a formula that automatically adjusts the smoothing factor based on the distance between the price and the average.

Historical Context

Published by John McGinley in the Market Technicians Association Journal (1991), the Dynamic was created to be a "market tool" rather than just an indicator. McGinley argued that moving averages should not be fixed to a specific time period because the market's speed is not fixed.

Architecture & Physics

The MGDI formula is unique. It looks like an EMA, but the smoothing constant is dynamic and depends on the ratio of Price to the previous MGDI value.

• Price > MGDI: The market is speeding up (or recovering). The denominator grows, slowing the adjustment to prevent overshoot.
• Price < MGDI: The market is falling. The formula adapts to hug the price without breaking.

Mathematical Foundation

\text{MGDI}_t = \text{MGDI}_{t-1} + \frac{P_t - \text{MGDI}_{t-1}}{k \times N \times (\frac{P_t}{\text{MGDI}_{t-1}})^4}

Where:

• N is the period (roughly analogous to an EMA period).
• k is a constant (usually 0.6).
• The term (P_t / \text{MGDI}_{t-1})^4 is the accelerator/decelerator.

Performance Profile

This is one of the fastest adaptive indicators available.

Metric	Score	Notes
Throughput	[N] ns/bar	Scalar math
Allocations	0	Stack-based calculations only
Complexity	O(1)	Constant time update
Accuracy	9/10	Hugs price closely without breaking
Timeliness	8/10	Accelerates to catch up to price
Overshoot	9/10	Specifically designed to minimize overshoot
Smoothness	9/10	Visually pleasing, organic curve

Validation

Validated against Skender and Ooples.

Library	Status	Notes
QuanTAlib	✅	Validated.
Skender	✅	Matches GetDynamic
Ooples	✅	Matches CalculateMcGinleyDynamicIndicator
TA-Lib	N/A	Not implemented

| Tulip | N/A | Not implemented. |

Common Pitfalls

1. Not an EMA: Do not treat it like an EMA. It does not have a fixed alpha.
2. Period Meaning: The "Period" N is a calibration constant, not a hard window size. An MGDI(14) does not "look back" 14 bars in the traditional sense; it's just calibrated to that timeframe.
3. K Factor: The constant k=0.6 is standard. Changing it changes the sensitivity.