Charts and key technical levels suggest that investors should not sell shares of Equifax  (EFX - Get Report) on Friday weakness, but should instead buy weakness to my annual value level of $119.51, which has already been tested once as the stock traded as low as $117.25.

Shares of Equifax plunged 15% to 18% at the open Friday, after the credit-reporting company disclosed that it experienced one of the worst data breaches in history between mid-May and July. Cyber criminals gained access to personal data on up to 143 million customers.

Technically the stock set its post-election intraday high of $147.02 on July 26, then the company discovered this breach on July 29. What took Equifax so long to disclose the breach?

The weekly chart warned that something was amiss, as the stock began to have declining momentum during the week of Aug. 11. The worst-case scenario is that the stock eventually falls to its 200-week simple moving average of $103.12, which is also considered its "reversion to the mean."

We will look at the daily chart and its Fibonacci Retracement levels from the stocks post-election low of $110.87 set on Dec. 1 and its post-election high of $147.02 set on July 26.

Daily Chart for Equifax

Courtesy of MetaStock Xenith

At Monday's open, shares to Equifax plunged below their 23.6% retracement of $138.43, their 38.2% retracement of $133.16, their 50% retracement of $128.90 and their 61.8% retracement of $124.64, which has been tested at Friday's high.

Equifax closed Thursday at $142.72 and opened at $121.82 Friday morning, down 15%. The stock traded as low as $117.25, down 18% before noon.

At Friday's open, the stock was still up 3% year to date and up 9.9% since its Dec. 1 low of $110.87. The stock opened 17.1% below its post-election high of $147.02.

Investment Strategy: Buy weakness to my annual value level of $119.51. Sell strength to my semiannual risky levels of $145.78.

Note that if the stock closes above its 61.8% retracement of $124.64, the next target is the 50% retracement of $128.90.

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This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.